I believe this is a sensible course of action and a natural evolution/pivot/hedge of making use of an existing social network/community to satisfy a couple goals (for Google):

- Have a general-purpose social network, but bootstrapped from a userbase where it can be welcome rather than vilified (unlike Buzz, Google+, etc.)

- Stop the slow trickle of (duplicative, cross-pollinated) traffic from the Youtube community to other platforms, because their own platform is lacking (they needed Twitter-like micro-announcements badly)

- Better arm themselves against other sites that have done video from the beginning but are expected to develop a similar direction (Twitch)

- Better arm themselves against other sites that didn't start off with video but have branched out into it (Facebook & subsidiaries, Twitter & subsidiaries, Tumblr, most other social networks these days)

Hopefully this also stops the (IMO, frankly irrational [1][2][3]) speculation that Twitter will be bought out by Google.

[1] https://news.ycombinator.com/item?id=12418727#12420732 [2] https://news.ycombinator.com/item?id=12083561#12083975 [3] https://news.ycombinator.com/item?id=11913828#11914620