This is my take on things but happy to hear other views. Google (and Facebook) are advertising based so try thinking about it in terms of how well they know their users.
The better they know their users, the more specifically they can target ads to them. For example, if I wanted to reach 20-somethings, who really like pizza and motorbikes, travel widely and currently reside on the west coast, FB can put ads in front of that group (I don't know if Google can, but I assume it's more tricky for them).
Since there are likely a large number of people that use the web while still logged in to FB, then FB could feasibly start offering to place highly-targeted ads out on the web at large. This could obviously have a dramatic effect on Google's dominance.
In other words, it's not just social per se, but being able to learn more about users in order to ensure that they can keep their hold over advertising revenue. Once I looked at it this way, Google's 'need' for social made more sense.
To test the above, we could look at what Google offers in terms of demographics/interest filtering when you buy/place ads around the web. If the ability to refine who sees your ads is really part of the plan then there should be signs that it's happening. I've never used any of Google's ad stuff so I wouldn't know where to look.