Good point. I have given a lot of thought to sustainability with Facebook Ads, and my optimism follows the basic data of my experience ever since it opened up.
Originally, low CPC's were quite commonplace. I could advertise something and get a pretty good rate - $0.10 or so - and make some really good money. Over time, the base CPC has risen across the world. Right now, you're looking at $1.20 or so just to hit the US untargeted (this isn't the suggested bid, this is actually running all-US ads).
What is really telling however is CPC's on very tight interest groups. Let's say I target a set of interests in a city that gets me about ~2k people in the profile. It used to be, even a year ago, that I'd pay about $0.80 per click. Today, you'll be lucky to find $1.30 on the low end. As niches get narrower and people get better at mining interest, the CPC climbs. Long term, sustained CPC trends are clear indicators of increasing demand and the ROI to sustain them.
Why this isn't talked about so much has to do with barriers to entry. I'm telling everyone here a lot about what I do without revealing the finer details. Also, I would say that as with any venue, some products do better than others. You can sell big-ticket stuff on FB, you just have to go about it much differently. You can lose a lot of money on Facebook quite easily, so how you approach the market is a big factor.
That said, the secrecy surrounding the success FB advertisers like myself are having is a much stronger indicator than anything that is said publicly. Will it keep FB from missing earnings targets? Probably not, but I remain very bullish on where they are going and what they have to offer.