I’m at SDSU Library (555 Campanile Drive, San Diego) w/ 4 others.
    1:41 PM Sep 9th via foursquare

    Getting better acquainted with this lovely auditing textbook (@ SDSU Library w/ 2 others)
    11:06 AM Sep 9th via foursquare

    Study time… (@ SDSU Library)
    10:59 AM Sep 7th via foursquare from SDSU Malcom A. Love Library, San Diego

    I just ousted Timothy P. as the mayor of SDSU Library on @foursquare!
    12:41 PM Sep 7th via foursquare

What do these four Twitter tweets about the SDSU Library have in common? The posters all announced they were at the library via Foursquare.

Foursquare is a growing location-based social network that helps people connect with friends using GPS via their mobile device. Say you arrive at your favorite coffee shop and want some company. You “check in” on your cellphone, and your friends can see where you are on a Foursquare map. And by the way, you earn points for checking in from various locales, so being mobile and adventurous are pluses.

One of the cool things about Foursquare is you can integrate it with Twitter. When you check in on Foursquare, you have the option to tweet it out on Twitter (say that fast five times!).

And no, as far as I know, we have no elected officials in the library, mayors or otherwise. What Timothy P.’s vanquisher is referring to is, when you check in at the same location often, you become the “mayor” of that spot, and you have to defend your title as part of the game.

Foursquare isn’t the only location-based app; Loopt, Brightkite, Whrrl, and now even Facebook offer similar services. If you’d like to learn more, I’ve included a couple of links to articles on Mashable that can explain location-based social networking far better than I can. So the next time you tell someone you’re “going to the library” instead of helping them move or do the laundry, you can prove it!

More information on located-based social networking:

Beyond Foursquare
Foursquare: Why It May Be the Next Twitter