I think you are a little bit wrong about social. E.g. I know MySQL developer who works for Facebook. How MySQL can be social? The only thing Facebook brings to this is how much you should scale but the same is for Google.
While I agree that choice must be based on desired position/knowledge or specialization that author wants to acquire. It is completely possible that neither Google nor Facebook is right for you because there are limits how much you can do there. Don't forget that you will need to compete with the best what is good for your education but it might be possible that it is not the knowledge you want to acquire.
Well, consider working on search and thinking "geesh my dream job is not to make search better. I don't like AI. I want to work on building automation tools for the build system. Maybe the search part can help me understand how to make build system more intelligently, but still, why waste 18 months of my time? Here is GitHub / Bitbucket hiring me doing what exactly I love to do! "So then you would end up going there.
Okay. Maybe I am a bit cynical about Facebook's work. But the reality is a lot of what they do is toward the scale of social media and on the surface people think "okay new feature from Facebook. Cool Story. Look at Google, more interesting shit. Even their DB work is used multiple domains, not just for social aspect. Because Google is not just a social network company. That's the point I was trying to make. Maybe it's a bit cynical, but it's true. If you go to a different team at FB, still social network? Damn. Let me do something else already.