I can't say anything more about Avatar and Titanic than has already been said except this: they are as thin as paper and manipulative as hell. Very very overrated.
Have to also reinforce that The Social Network is way overhyped and overrated. This is a TV-movie-of-the-week script shot for the big screen, with some nice directing, editing and cinematography. That's it. I've seen it twice and I'm still wondering what the hype was about.
Finally, and absolutely, my pick for the most overrated movie ever is the Academy Award winning Forrest Gump. What an overwhelmingly wrong choice that was – it beat Pulp Fiction and The Shawshank Redemption, for pete's sake! If life is like a box of chocolates, this movie is a box filled with candy corn. Empty calories, corny sentimentality and an absolutely awful message ("success is luck"). It's just average at best.