I just now chose to search IMDB's Facebook page for "Star Wars The Force Awakens" for a recent huge movie, using IMDB's supposed "replacement" to these forums. It ought to get no better than this. Top three results:
1. Which film do you think will be the next $2 billion dollar movie?
2. John Boyega to star in Pacific Rim 2
3. Daisy Ridley to star in novel adaption Chaos Walking
None of these are even about the movie I searched, much less a focused discussion! It's completely useless. But that's not because Facebook's search engine is bad. It's searching what has been posted. No, it's because Facebook is not a search engine for catalogued knowledge to begin with. Why? Because Facebook doesn't even contain organized information. It's the wrong tool for this job. It's like going to a pub to extract information rather than going to a library.
This is in extent a problem I've felt has grown worse lately. The big social networks are eating up the web which is not only a loss due to consolidation and reliance on what the giants decide to do (a ton of eggs in very few baskets), but because social networks are tuned to be live or at best "the fresh of today". That works when chatting daily with friends, but not to look up thoughts and opinions on a movie from 1995, or anything else for that matter.
There is also a conceptual difference here, even if social networks did catalogue information to be browsable. They usually work as advertisement platforms and people write reactionary comments, while on a message board people go there in order to indulge themselves in a discussion or get questions answered = much higher discussion quality. Even if the IMDB boards weren't exactly always top quality, they were often much better than the swamps of Facebook.