While filling out my profile it struck me that I'd love a friend finding service: Find me programmers within 50 miles of my stated location who share interests and experiences in some of the many tags I chose for myself. I'd like to meet them and have a beer.
The key here is that StackExchange has a high usage rate amongst programmers and it allows me to register interests in technologies that I don't have public source code for.
Facebook has a nice social graph but no angle on programmer socialization. Github's API can let you infer which open source languages I dabble in, but not at all what I do at my day job. Good luck to you though, I'd love to see the product.
