Back in the proper days of Usenet, people tended to write long posts, and even if they weren't long they had big headers and got distributed all over the place, so the cost of being spammy was non-trivial. It was not Twitter.
So, there was fairly heavy social pressure not to write 'me too!' e-mails. Which people inevitably did, and were told not to. They added nothing to the conversation, you see. Well, kinda they did, in that they could emphasise consensus, but the cost to do so was too high, so everyone pretended such 'aol' posts were worthless in the world of weighty conversation.
Come Facebook, posts are much, much shorter. On the other hand, people still want to agree with the poster. You don't even need to comment to agree - there is a 'Like' button. So, rather than creating a social solution to a technical problem - the Usenet approach - the technology is modified to meet social needs.
While it's traditional to bemoan the move away from Usenet, in this way, I think the change is an improvement.
And then there's Twitter, which is just all spam. ;)