Hi5? Probably not...unless Yahoo wants to deepen it's reach in Latin America...
Perfspot? Their traffic is down & to the right...(per quantcast).
Bebo? Probably not (besides, they'd have to uncouple it from AOL, etc).
StumbleUpon? I think eBay learned their lesson on that one.
Facebook? They don't have the ~8 billion, I think, that Facebook would want them to have to pull it off.
LinkedIn? Perhaps...and, Jeff Weiner is President there, former EVP of Yahoo, etc...could be interesting & a great tie in with their Zimbra line of products. *that* would make for a compelling "menage-a-trois" between Zimbra, LinkedIn and Hotjobs...if they'd put them all together.
Ning? That could be interesting...however, I'm afraid it'd end up "geocities 2.0" in reality...eg, neglected & shut down after spending far too much money to ever reap a profit.
Piczo? Not enough traffic imho, to make it interesting.
Fubar? Probably not...uniques is also around a million per month, which makes it very unlikely.
Tagged.com? Perhaps, there is some traffic there...but, would it really add value?
To me...I can't see any company off hand that "fits" with what Yahoo needs to do, which is to start leveraging (flickr, mybloglog, delicious, etc) it's *current* social assets in a way that makes sense...