I have been doing paid advertising with a few million dollars budget, with a strong conversion focus mostly on google, social web sites and content web sites. Social web sites have low relevance, low CTR for most purposes. However they are an ocean of impressions. Facebook has a chance to replace the advertorial parts of cheap TV non-prime time. If you use it properly you may get conversions though a bit lower quality than search and content.

So Facebook has the following positive attributes; huge number of impressions or minutes spent, good demographic targeting. And finally yes, if they launched a search engine that made sense, they may increase their value to advertizers. But I don't know how.