Loopt, Gowalla slain. Foursquare to follow.

Transient

Loopt is gone. Acquired, but really, it just failed. Honestly I've been a huge pessimist of all the "location-based social services" and I predict that Foursquare is going to go a similar route unless it essentially morphs into something else. People tell me "oh well naysayers like you said the same thing about Twitter! And look at Twitter now!!!1one". 

But there's a huge fundamental difference between Twitter and these services:

Checkins are just not interesting.

Meaning, I can't read a list of checkins and find it valuable whatsoever. They're just dry GPS coordinates with a little bit of context about people I don't know or care about.

On the other hand, I can read tweets all day long because they're often packed with amusing comments, important news, or links to an even richer medium: blog posts. Twitter is your "what's new on the web" feed, displacing RSS and newsletters and other ways to ping you. Moral of the story: because Tweets are actually fun to consume, Twitter will enjoy longevity as a medium.