The guys from TechCrunch claim Yahoo is working on a social network called Mosh:
Here’s a juicy tip - we’ve been hearing about a new Yahoo social network initiative called Mosh, which is at mosh.yahoo.com but can only be accessed from inside the Yahoo offices. If you happen to be using the guest wifi at Yahoo, you should be able to access the site, although this may be shut down soon.
It’s likely this would replace Yahoo’s 360 social network service, which has never really gotten traction. The existence of Yahoo Mosh also most likely puts a bullet in any further speculation that they are in acquisition talks with Bebo, a somewhat unsubstantiated rumor from May. Last year Yahoo made a serious effort at acquiring Facebook but the deal was never closed.