If the deal goes through Yahoo would now own the Tumblr audience, so Yahoo would have a product with a young audience, the audience Marissa Mayer wants for Yahoo, however, how would they move that audience over to other Yahoo products?

An acquisition of Tumblr seems very much like the acquisition of reddit by Advance Publications. They both have huge audiences that are passionate and growing fast but have great difficulty monetising their audiences effectively and are providing no real value to the parent company (other than the potential for the sites to become profitable). If Yahoo bought Tumblr how would they ever convert the audience into Yahoo users?

I guess it comes down to: what value is there to any company in "owning" an audience if that audience has no interest in becoming the audience of the parent and will actively resist it? People would get very angry if AP started pushing their ownership of reddit onto reddit users, having Wired articles automatically frontpaged, things like that would drive everyone away, how will Yahoo avoid that?