Earlier quoted context omitted.
What would be the point of building a social network for Yahoo? Better engagement? I'm pretty sure they always have been a destination for much of their audience (as opposed to depending on social sharing to get eyeballs, like, say BuzzFeed). So I'm not sure they'd have a whole lot to gain from it. Of course, their audience probably will spend more and more time on social networks, so stemming that tide somehow might…
Why does the world need yet another social network?
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.