I believe JimmyHAD has it right. FB provides frequent new stimulation and we all like that.
At the same time I believe that FB is a "fad/flash in the pan/temporary type of thing. It's weakness is that their is no depth on a subject matter and no permanency of knowledge. Therefore I think that for the subjects where depth is important the forums will always be the primary resource.
At this point in time I am actively "unfollowing" groups and people on FB because it is so superficial and I have to many of them. There are other things I want to do with my time, one of which is getting back to contributing to SRP.