They also claim not to use Chrome data. If they are being completely upfront about Chrome (and I'm not saying they are) and they can crawl Facebook as you say, then Facebook becomes just another website for link building.
But it seems like Google has much more data about links these days, such as which ones do people click, how many clicks do those links generate, etc. and that would have to obtained from something like a browser.
So whether Google is using Chrome data or not, it seems to me that Facebook is just a website like any other where you can benefit from getting the right links naturally and be penalised for links that aren't freely given, rather than a site that has some unique special powers you won't find on other busy, popular websites.
Google+ is different because they have more in depth data so potentially you have more to gain if you do things the right way but more to lose if you try to game it.
Just thinking aloud. I haven't made my mind up about the value of social yet or whether the returns justify the effort required, but I am now beginning to think that social sites are just another website, so the value they could bring depends on the same factors as any other site you might want links from. This is becoming more straightforward in my mind now, but maybe I'm over simplifying?