Tumblr+flickr+mail does not even get close to being a social network with an existing userbase to rival Facebook.

What it nets you is three independently operated (non integrated) software-as-a-service offerings, a fragmented user experience, a branding nightmare and a userbase that is a small fraction of Facebook's once you've done the undoubling / removal of dead accounts.

What other businessmodel than advertising would you suggest Yahoo should use?