I noticed a curious statistic today: my Twitter network is almost three times larger than my Dopplr network. Inquiring minds want to know: how could that be?

Let’s ignore a couple of perfectly reasonable, but uninteresting, possible explanations. Maybe the “sample size” of my networks is so small that any difference between the two is statistically meaningless (I don’t have the armies of devoted fans that you’ll find on Scobleizer’s or Guy Kawasaki’s networks). Maybe it’s just a matter of time; maybe in another couple of weeks, months, or years, my Dopplr network will catch up to Twitter. Maybe Dopplr has been lagging because it was a “closed” beta service until last month, and new members could only join if they received an invitation from an existing member. Maybe, maybe, maybe …

But I think it’s more fundamental than that — and while some of the differences may be simple and straightforward, I think we’re going to see comparisons like this (including more “obvious” comparisons like Facebook versus MySpace) that will illustrate how subtle and nuanced all of these social networks really are. Indeed, this may be “old news” to sociologists and psychologists; but for the geeks (like me) and the average participant of these networks, it may come as a surprise.

To illustrate, let’s start with a simple, straightforward, but reasonably fundamental difference between Twitter and Dopplr. Dopplr is for “travelers” — i.e., for people who do enough traveling that they’re likely to be know friends, family members, professional colleagues, and co-workers who also travel — and who have their own “war stories” about flight delays, lousy hotels, weird taxi drivers, great restaurants, exotic cities, etc. I’m no longer as much of a “road warrior” as I was in the 1970s, 80s, and 90s, but I’ve currently got 13 trips, to four different countries outside the U.S., scheduled between now and the end of 2008; I expect to have a dozen additional trips by year’s end. And through the normal, day-to-day interactions of my business life, I’ve come to know dozens, if not hundreds of other people who travel at least that much, if not more.

Twitter, on the other hand, serves a much larger spectrum of people. I don’t know how many of the 744,042 Twitter members (according to TwitDir on January 7, 2008) are “professional” people, as opposed to students, homebodies, or “just plain folks” — but I doubt very much that a large percentage do a great deal of traveling outside their normal commuting range. So it should be no surprise that, in the normal course of events, we Dopplr-oriented “travelers” would accumulate an even larger number of non-traveling Twitterers with whom to interact.

But here’s a more subtle example — and one that may or may not be at all significant. Most social networks — not only including Twitter and Dopplr, but also Flickr and Facebook and MySpace and several more — make it fairly easy for members to add a photo, or “mugshot” of themselves. Some are very pedestrian, others are obviously staged or contrived; some are whimsical or even abstract. But in the absence of a user-supplied photo, most social networks will supply a “default” icon; in the case of Dopplr, it’s a simple sketch of a traveler’s suitcase. This isn’t a big deal, of course, but it should be noted that while finding and uploading a suitable mugshot takes a few minutes of work, it certainly isn’t rocket science … and it shouldn’t pose any technical problem for someone who has figured out to register for one of the social networks in the first place.

That being said, here’s an interesting comparison between three of the social networks I subscribe to. Of the 113 current members of my Twitter network, only five have anonymous mug-shots; I clipped out a screen shot, so you can see what it looks like:

By contrast, more than a third — 17 out of 45 — of my Dopplr “fellow travelers” have anonymous suitcase-mugshots; here’s what it looks like:

Again, I realize that my own little networks might be statistical anomalies — but I’ve looked at the mugshot galleries of other Twitter members and Dopplr members, and it looks like the same pattern repeats itself fairly consistently. Does this mean Dopplr people are lazier? More secretive? Less technically proficient? Are Twitter members more gregarious? More narcissistic? More flamboyant? Who knows …

When I posted a short “tweet” on Twitter this morning about the substantially smaller number of people on my Dopplr network, I got a few responses suggesting that people might be reluctant to expose themselves to explicit invitations for “get-togethers” on Dopplr. As one Twitterer (whose anonymity I’ll preserve here) said in a short tweet back to me (and everyone else on our respective networks), “perhaps I’m being sensitive but I know people read, skip, comment, ignore my tweets; an invite to connect personally is different.”

Actually, Dopplr is far less “active” than one might infer from this tweet. Indeed, all you do is identify the starting date, ending date, and city associated with each of your business trips; you’re not required — or even encouraged in any explicit way — to identify what you’re doing on the trip, which client you’re visiting, which hotel you’re staying in, or anything else. Indeed, there’s no obligation to list every trip; if you’ve decided to visit Kabul to spy on local terrorists for the CIA, you can leave that trip off your Dopplr itinerary.

Once having listed all of your trips, Dopplr then compares the data with similar data associated with everyone else on your network — which, by the way, consists only of people with whom you’ve explicitly indicated you’re willing to share such data. So the vast majority of Dopplr members have no idea who you are, which trips you’re taking, or anything else. As for your “fellow travelers,” all they receive is an email message from Dopplr that tells them of a “travel coincidence” — e.g., “You might like to know that you and Joe Shmoe are both going to be in Paris on April 12-13.” What you decided to do about that — or not do about that — is entirely your business.

Of course, that means you might get an invitation from someone you’d just as soon not see — e.g., maybe you’d rather spend your spare time in Paris on April 12-13 visiting the Louvre, rather than having a boring dinner with Joe Shmoe, even though he’s an important client. Well, that does require some diplomacy; and the tweets I got from fellow Twitterers today suggests that a lot of people would just as soon avoid such situations altogether. So they’ll happily Twitter with each other, because they can ignore the individual tweets; but they don’t want to be put in a position where they might actually have to say “no” to a fellow Dopplr traveler who has noticed a “travel coincidence.”

I don’t know how important this particular example is; but I suspect we’re going to see a lot more in the future. And the more we understand about these subtleties, the more useful the social networks will be for us …