In Facebook all you do is post lame stupid statuses. In twitter it's basically the same but with celebrities. In instagram you just post stupid pictures. They're all lame to me. (Even though i have a facebook and a twitter.)
It depends on what you want. For me Facebook works best. I use it to keep in touch with family and friends since I am so far from them. I get to know what is happening in the day to day.
I don't have the other two but my impression of twitter has not been good. People posting their random thoughts without thinking thru what they are putting out for the world to see. Some people really need to think before the text.
I recommend twitter its an awesome way to connect with celebs and see others tweet and retweet. I really get inspired by some peoples tweets plus I'm stalking celebs all the time so if you got twitter follow me @masarajeel