Ah man. I wrote a long-ass reply, but I lost it when trying to post. I'll try to quickly hammer out a readable version.
"What do I want a social network to do for me?"
The only reason I ever actively used Facebook was to look up info on my friends. So, I suppose I want a passive "black book" more than anything. A universal contact list.
Let users enter their data for defined fields (personal email, professional email, personal phone, pro phone, address, IMs, popular sites) and then them also define their own fields for inclusion (in case I want to let people know a particular forum or new service I spend time on. It's a good way to see what your users use.)
Then display it in a very minimalist way. Ever used Passpack? It lists my services, and has a grid of icons. So, list my contacts, and their info. When they don't have that info, leave the grid blank or grayed out. When they do, offer a clickable link for the site/service. (A link to IM them on that service, or to their profile on a site.)
Also let me add labels to each service, and to my contacts. For example, my personal email would have "friends", "online friends", and "family" labels, allowing each person I assigned those labels to see my personal email.
And bonus points if people sign up with their email addresses or phone numbers, rather than a site-specific username. It's silly for people to have to jockey for those on another service, relegating themselves to bigThunder1452 or the likes.
Asking users for the passwords (but not storing them) to confirm they own the accounts they list is nice, too. If a user hasn't logged in within the past month, ask them to confirm their data upon login.
With a minimalist design of a banner at the top, a list of recent contacts at login, and ways to list people by label, alpha, letter, or by account type, Throw an ad at the bottom or on a side.
I'm betting many business professionals would keep that tab open like I currently do with Gmail.
Hell, I've even thought of a name. Call it FriendInfo, and use FriendIn.fo, with, of course, purchasing friendinfo.com and friend.info as redirects.