It's little different to myspace so far as i've seen, they promote different features in that myspace was always about media and visual where as facebook went for clean (went, not any more, now it's a "X likes" shit hole of a site) and focused on events and groups.
When i was 18, myspace was all it really was, bebo to a slight extent and facebook was just seen as a bit of a joke, most people saw it as just a spivvy mysapce. With regard to numbers of users, well it might be to do with the ease of access for ost people these days. When i was using myspace, i'd either have to wait to get home at the weekends, or jump onto the school bursars internet connection, these days facebook gives you starting prompts. action 1 for getting started is a photo, action 2 is activating your mobile phone. I personally think give it some years and something new and better will replace it, be it based around 3D moniters and the added depth abiliyt it would give or i don't know.
As to the most popular thing ever, well a. there are lots of these, such as the ipod (apparently) and b. Popular does not mean it is good in the grand scale of things. Facebook has turned 500 million+ people into frigging tools, i mean at first it was great, uni social events would nbe organised and invites sent out etc, now it is farmville, 3rd party like websites, like pages that must be clicked on before you cans ee what was to like, people posting pointless status updates tailored to simply try and grab as much attention despite it making them look like a twat
