Facebook has kept it simple and added ads in smart ways with a nice clean layout.it's a shame though i have never used it but they had alot of users at one point. but the social media market is a very risky market indeed though if you look at it most of them have a life span of 3-5 years i am surprised facebook is alive today and i got to give credit to their ceo and his amazing team.
Twitter on the other hand has too much sponsored and is all over the place from the original.
Instagram is slowly but surely becoming the same still a good app for the time being though.
Its crazy to think the next Facebook might have already been 'thought up' and some little guy (or girl) is sitting in a dorm, their garage, their basement etc. Working away on it already, Wouldn't mind being one of the first to invest lol if it is the case.