I like to think of FB as a young Google. People keep questioning how much more it can earn in ad revenue. Maybe they've peaked (assuming no further user growth). But Facebook will create new revenue streams, in my opinion. They're building huge data centers in Iowa and Sweden, for starters. And a new campus is imminent (I know this sounds superficial, but it'll add to innovation, which will, in turn, add to the bottom line).
And I believe in Mark. He's smart and more interested in bettering the company than he is in earning money. FB is a worthwhile growth investment.