I'm defining "social network" pretty broadly, including websites that have a strong social component, like HN, reddit and imgur. These kinds of online communities tend to make people happy, and they let them form friendships and exchange ideas. If a new one emerges then it's probably making some critical mass of people happy.
I would guess that Yahoo's audience might not have a ton of exposure to all the different social networks out there on these days, and they might appreciate a stronger community aspect to Yahoo. A Facebook clone wouldn't make sense, but something that gave it more of the flavor of imgur or HN might. Though I suspect that would be really hard to pull off.
It wouldn't be the bet thing for Yahoo to focus on, but it wouldn't be the worst.
They've done Buzz! (very Digg/reddit/HNews-like) and a handful of other social net products since their founding. Executing that well -- like with most things -- takes focus and willingness to get behind something particular, rather than everything, and Yahoo's often spread their peanut butter a little thin. http://www.wsj.com/public/article/SB116379821933826657-0mbjX...
