On the subway ride to the airport today in NY there was an ad for a new construction project at Fulton St. It had a Twitter account for status updates, but no Facebook account. Not sure why, but if you had asked me to bet whether Twitter would be mainstream, just a couple of years ago, I would have bet against it. And would have lost.

Another angle -- LinkedIn is for business contacts. Facebook is for connecting with classmates and friends from days gone by. Twitter is for news. What is Foursquare for? And then what is Facebook's checkin service for? I mean, when it all shakes out. It can be hard to forsee.

But I got a glimpse of the utility today when I checked in at the Seattle airport. I could see 20 people I knew who had checked in earlier today. Presumably most of them are still here tonight. It gets me thinking. I wonder why this person is here. I have a theory why this other person is. And there were some surprises, and I could see some meetings that were probably happening just based on the connections between people who are here and those who live here.

Back in the old days when I'd arrive at an Esther or Stewart conference, first thing I'd do after checking in is review the list of people who were also there, and plot out a route through the network of people who were there. I felt that way today when reviewing the check-ins at Seatac. And realized that social networks of today make good models for the face-to-face conference networks we used to depend on for connecting.

Does Foursquare have the critical mass to pull off what Facebook just did for me today? Do they have that focus? Are they trying to position as the business-based social place-aware net? And LinkedIn should get this feature soon if they want to fend off an intrusion by Facebook. Hey it may already be too late. Or I may totally not understand what's going on.

 9/8/2010; 2:13:56 AM.   .