Yesterday in footnotes, I mentioned that WCUS is promoting the conference with Facebook ads. I got word that it is being done with official blessing.
Furthermore, I learned from Andrea Middleton (who runs WordCamps broadly, but wasn't involved in this ad buying situation) that a few camps have tried buying Facebook ads before, with little success. They typically recommend not buying Facebook ads, but because of their lack of benefit, not a rule. You can see notes on promotion on the Planning WordCamp site, though it doesn't directly address paying for promotion. What many WordCamps do that does work well is manually request local influential bloggers and more general news sites for articles or announcements of their WordCamps.
Promotion and advertising may end up a topic at the Community Summit, if you're into this kind of thing. But for your own WordCamps, there isn't a rule against advertising, but you'll want to only do something if you either need it for your local audience, or it makes sense financially, given the relatively tight nature of WordCamp budgets.