I don't buy that MS and FB are closing the web. First, I don't see how the Yahoo! Search business and the Facebook buy are tied together. MS could use their own search engine to crawl Facebook's data.
And on a side note, I don't know that Facebook is as sticky as some people think it is. I like it, but I think that there are too many moving parts to say that it is going to hold it's position long term.
So even if it happens, I don't think it's that huge of a problem for Google.