But you need to know these people. In a forum you can ask a question to the world, in your Facebook to your friends only.
A forum can not be compared to social media. The younger generation will also have demands which only a forum can fullfill.
Say you want to know which dildo is best, you will ask that in Facebook for everyone to read (including family)? No, you will ask it in a forum under an anonymous name. Forums fulfill a need, also for the younger generation.