first Check out www.solderbynumbers.com
for some good ideas and some things to AVOID. They have some serious issues when it comes to their design software and sharing of circuits.
I think its a good idea. Not the next facebook but there might be a market for it.
Here is what I would like to see:
1. A tutorial link system. Not host them just link to them. So I can post a list of tutorials I right or have used for others to see.
2. A way to share circuits and code. Maye even a snippet library to help sharing code
3. A way to find local people with similar interests.
4. A group function to allow people to form groups based on interests or location
5. Robot galleries to show off in.
Probably a hundred other things as well but the big thing for me is to work on functionality instead of adding features. If its got a ton of features and a ton of bugs then you end up like solderbynumbers.com Great idea they just have poor execution.