However, what happens when your friends decide, "screw this, I'm out"? All of a sudden, those connections that you were paying for aren't there anymore. At best, I'd say Facebook could get 50% of its users to pay for its service. Once half of those connections are gone, the service isn't worth as much to you. Likewise, as users leave, so do their photos. Again, you're in a situation where Facebook's value is being lowered for you.
That's what's so hard about pricing. Facebook is worth $10/mo to you. It isn't worth that much to others. However, it's only worth $10 to you because of the other people on it (many of whom wouldn't pay that). Which means that if Facebook starts charging money, suddenly it isn't worth $10 to you because much of the content from other users (their profiles, your connections to them, their photos) is gone.
There's a critical mass issue. Even if you say something like, "well, they could charge for premium stuff like the ability to say who gets to see your profile" which is free now. In that case, lots of users won't post content as freely and the service looses some amount of its value.
So, Facebook really doesn't have the opportunity to charge people because once it does, the value goes down as the more marginal users are no longer there.