There are a few key differences. Facebook was a direct attack on myspace but it still doesnt have any direct competition. True youngsters are going to better platforms like snapchat but from what it seems, instagram is getting good. Facebook has a lot of tools under its belt - whatsapp, instagram, facebook for work and then the marketplace which is crap but can be scaled.
Add to that all the grown ups still to get onto fb in developing and underdeveloped countries. I think there is still growth left for fb. The myspace moment is far away.