Why is it considered that if you sell your company, you're not in it for the long run? I get that Mark doesn't love money but he's not doing FB for charity. I'm not sure why he didn't sell to Yahoo for $1B or to MS for $15B but the only thing I can guess is he wanted more (money?) out of it.
In other words, he feels/felt that facebook is worth a lot more than $15B. I don't use facebook but I find this to be the most admirable part of facebook's evolution. For someone in their early twenties to have that sort of conviction about the worth of their product is remarkable, don't you think?