Recently I ran a campaign on Facebook to promote one of my gigs. My budget was in the hundreds, not much. Sadly none of it convert into purchase but I have several likes on my Facebook page… The pay-per-click rate was pretty expensive since I targeted the US. So if you have a $5 gig I wouldn’t recommend advertising on Facebook
In any case I look at it as an experiment. One useful info I learned is that Facebook also rate your ads from 1 to 10 that estimates how well your target audience is responding to your ad. My score was pretty low so I knew I was not reaching the right audience so I stopped the campaign.
So the trick is for advertiser to find the right audience then from there create a custom audience to get more targeted traffic. I’ll keep experimenting.