Is google intentionally lowering the rank of facebook pages? You search for something like "iphone" and you don't get any pages about the iphone on facebook. You get sites like mashable, engadget, etc, but no facebook pages? Facebook is one of the top sites, so why wouldn't it have a high ranking on practically every query? Is Google intentionally lowering the rank of Facebook (and twitter's tweet pages for that matter) because they're big competition on the web for Google?
I have no inside knowledge and I speak only as a research student working on search engines, but my guess would be that Facebook and Twitter don't allow their pages to be crawled[1]. If I recall MS had to strike a deal with both of them so that Bing could use them in their results. Another "problem" is that the notion of document relevance varies from a "standard" web page to a social network[2], and mixing them up on the same result page could be confusing.<p>[1]: a good social search would need realtime access to the API of those websites, which is why you still see "some" content like FB profiles in Google's results.<p>[2]: what I mean is that you have additional parameters to take into account: incoming/outcoming edges, "status" of the author, additional metrics of popularity, redundancy of content... Social search is an entire research topic of its own.
I wouldn't say "lowering the rank" but Google does have special cases, white lists, etc. It'd be in everyones interest for them to factor out many Facebook entries except for Profiles, Pages, apps, and Facebook internal pages. Else, people would try to use FB posts to game the results. The same likely goes for tweets.