People enjoy this because it is: social, private & immediate
Sharing it: http://4sq.com/bwxGnW [aka http://foursquare.com/stats/jessamyn]
So this is what my title is about. In places where this sort of thing has caught on, places with ubiquitous good internet and a well-to-do population of smartphone owners, this is a lively back and forth. For a long time, this must have been last year, my twitter pages were all full of "I just ousted soandso as the mayor of such and such" and it took me a while to figure out what was going on.
And even now when I use it, if I'm around home, I'm more often adding venues than checking-in to them. I'm adding a dot on the map, and maybe something about them. I like to think I'm supporting the local BBQ place when I say "good BBQ even if it's in the parking lot of a gas station" Of course you can only add dots when your phone is working. And yes, I'm mayor of everywhere, mostly.
There are other ways people are using these systems, and these apps. Here's some show and tell. Links are on the credits page.