I thought of the very same idea, but never even built it, I still think it would be cool. The main problem is that you have to have enough people on the network to make it useful.
Yup, hence rkuykendall-com's comment. Also this is where good marketing comes in. I tried some free Google ad credit and got tons of exposure but few clicks. My next guess would be to try large metro areas and retain old messages in hopes to bootstrap some conversation.
http://en.wikipedia.org/wiki/Reed's_law