We used FB advertising for Mammoth VPS for a while; I have a little experience in it but not that much. On the whole, we found it less successful than Google. I don't know for sure why, but my theory is that people actively engaged in looking for something are more receptive to ads that might be related than casual browsers who just have related interests to what you're advertising.
I targeted the Mammoth VPS ads at people with a bunch of keywords in their personal info - so straight off the bat it was obviously a tough call, because I would guess the majority of people that know what a VPS is probably aren't fleshing out their Facebook profile.
Here's a snapshot of our spends (over about 2 months):
One thing I found really weird was the difference in displays between "Cheap Australian VPS 1/2/3". They were the exact same ad, just with a different discount voucher, to try to see if there's was any magic discount voucher that grabbed people. Unfortunately that experiment was rendered useless by the fact that Option 2 was displayed significantly more times than the other options, for no reason I could see.
Cost-Per-Click they want $0.01c, or a $0.02c CPM, so we'd split part way. My question is how does their CPC work - like, if I dump $5/day into CPC, are they actually saying the ad will run until it's received 5,000 clicks, regardless of impressions? Or does it also have a CPM ceiling?
AFAIK there is no CPM ceiling - it'll just run until you've exhausted your campaign budget. $0.01 is an extremely low CPC though!! So I would take advantage of it.
Overall I'm undecided of its value - we've opted out of using it for now in favour of solely Google applications. I really like the targeting options - I think if we were selling something that the average Facebook user was into (ie, something that the zombie horde on Facebook is more likely to have in their profile as "things they like", like Twilight movies or whatever) it would be a really valuable resource, but for our relatively niche service I think our target demographic is probably just not using FB in that way.