They each serve different purposes. Well, at least I try to believe that.
I have Instagram, twitter, and facebook.
Instagram is pointless in my opinion. Besides for the simpler interface to view photos, it doesn't create anything new that twitter can't handle. Twitter is even implementing those stupid photo filters, so I really can't understand why people would even want an Instagram. Though, there is that "community" argument.
The only reason Twitter is able to exist is because Facebook became too crowded. Back when you had 300 friends, it was fine to post stupid statuses. Now, when people have hundreds more, even thousands of friends...those statuses are considered spam worthy. Celebrities or V.I.T.'s (Very Important Tweeters--as Twitter likes to call them) also don't' want to have to link their Facebook's directly to fans so having another social media outlet that could be exclusive to statuses was beneficial for them. Bottom line is that Facebook provides every function you could want...and all these other social media websites only exist because of the overcrowded effect which made Facebook inefficient.