The FastForward Blog just published a nice article about the use of social networks in the enterprise, something I already wrote about in a previous post. While I agree with part of the article - I am expecting Sharepoint and its ecosystem to skyrocket in 2008 too - I'm more optimistic about the adoption of social networks.
I think no project can start without the need to solve a business problem, thus social networks will be implemented for a reason.
Cross selling or team building are good candidates, but it's likely that early adopters will want to target specific business problems, as Armstron says. I don't think we are going to see any "Facebook for the enterprise" implementation at all.
As for delivering value, it's all about perspective.In my opinion a social network won't do any good for the efficiency of a process. A social network can, on the long run, give you a significant boost on effectiveness: anyway, it will not be possibile to measure benefits but after months, if not years.
It's a big bet, but I'm pretty sure companies who are "all about people" (consulting firms, for one) will seriously start thinking about social networks in 2008.