Like I said, I think that Google is here to stay and it will grow and re-invent itself many times.
Nevertheless, Facebook search is useful and I can understand why people use it in addition to Google. They simply supply different kind of results. Facebook can provide an audience opinion and Google can help find professional reliable information. For example, while looking for a hotel or planning a trip somewhere, people would like to see comments of past guests and not only read the official information that a marketer put on the business's web page. On the other hand, while looking for articles on architecture it is more likely to find relevant results on Google.
I also don't think Facebook will replace gmail. People that say that remind me of the people who said that the fax will replace the mail, or that the e-mail will replace the fax. New kinds of media don't necessarily have to take the place of the old ones. In many cases the new ones are simply added to the list of things that make our life more comfortable.