There’s more to the game: special trips, featured spots, and item numbers, but there’s really just one other aspect that’s important: if your friends play, they can link to your account, and then you can see when each other check in.
That turns out to be a really interesting way to be able to keep in touch with each other, and reminds me of what I *used* to use Twitter for—to tell my close, real friends exactly what I was up to.