I think there's also the way the communities of the two types of enviroment grew. Most forums i know about start out as a specialised subject and then comes those same people using the general discussion. So these people who get along with their similar interests then share their opinions on broader subject matter. Similarly IRC is still very active, with groups of people joined as a group for a purpose using it for casual chat. Then steam groups (for example) can take it in the other direction where it's a far more specialised about only one video game for example.
Social networking on the other hand is just nabbing at people's desires for friends and attention, i don't know how to expand the point i want to make on this because i don't use facebook for more than event organisation. But i just can't comprehend how people end up telling 300~ people, most who they don't know, about jam and pickle sandwich they had earlier. Then there are apps which tell people which stories i've read, or what music people have listened to, and 'like' {current contraversial topic article} to seem like an intellectual when in reality they're doing just for more attention and self gratification.
I rambled a bit, but in short, facebook is useful for centralisation of friends/family and personal digital effects but it's some horrible blob now. And i hope forums and the ilk are here to stay in one form or another forever.