Caroline McCarty at c|net offers up a great primer on the challenges facing location-enabled social networking by looking primarily at newcomer (in private beta) Brightkite, old hat Loopt, and older and Google-owned but rarely heard-from Dodgeball.

Key challenges:

- open the network to all or just some - should the app run on all phones, even ancient ones with no GPS and just texting capabilities? should the app be free? should the app be restricted to certain carriers? should you do a private beta and potential limit networks at startup?

- managing privacy and lists of friends - how do you keep annoying messages out of your "inbox"?

- where do you launch/grow such apps? business-y Web 2.0 (Brightkite was launched there) or social SXSW (South by Southwest, an arts, tech gathering) where Twitter took hold in 2007

Definitely worth a read if you are trying to figure out this space!

Mashable introduces the startup GeoGraffiti which offers a now free tool to post a "voice mail" and associate it with a location (ZIP Code or lat/lon). It's accessible via the Web (Google Map) of phone. Bottom line from the "review": not sure what it's good for, but that's what we thought of Twitter.

If we could get the accuracy for the messages and use a GPS... I could see using it as a tool to create a "treasure hunt" sort of game or even audio instructions for a bike ride or run.