Obviously nothing is guaranteed, and I personally believe that they'll have trouble finding anything that justifies their valuation. But a lot of smart people think they'll figure it out, and are putting their money where their mouths are. They have some options, but we'll just have to wait and see how those actually progress.
Perhaps the key word is "better".
For FB as a company, they're probably better off delaying their IPO as long as they can. As a private company, they don't have to make their books public, and can continue to "figure it out" without extra scrutiny. Delaying the IPO also lets them promote themselves as a pre-IPO "startup", while I don't personally see the growth potential, and I think they've got a lot to do to even meet their current valuation, it could help sway some developers. They've got a long way to go to justify their valuation, and they can use all the developer talent they can get, so any little thing can help.
$6 Billion per year revenue
The billing systems (and customer support) you'd have to put in place for something like this would likely make it prohibitive. It's the flipside of "there's no such thing as a free lunch": there's no such thing as a free lunch payment (or pricing) system. Billing, collections, currency conversion, and end-user hand-holding ... it doesn't hold up. Advertisers are far fewer, find far higher value in the system, and on a per-unit basis are much more economical to tap.
Network effects are another huge factor. A large part of FB's value is that everyone's on it, and everyone's on it because everyone's on it and it's free.
You could conceivably come up with an end run around some of this, but it would take some creative thinking. I don't believe it's feasible to have the end users be the direct customers of such a product directly, though with a concentrating middleman, possibly.