twitter is getting more popular today but i would really say that facebook got to be the most popular.
My opinion twitter and facebook are two big giant of social networking. I like facebook much and i have a account in that it is more famous than twitter and easy to accesses so i preferred facebook.
Facebook is older but twitter is gaining popularity (the word tweet has gained more significance these days) but personally I like neither
Twitter has been gained quite popularity recently, I think it has already beated Facebook, becoming the most popular networking these days. Just ask who don't twitter these days.
In a way, its kind of like asking which is more popular, apples or oranges . Anyhow, according to Alexa, Facebook is #3 while Twitter is #14 so I guess the edge goes to Facebook...