If it's minimal cost and upkeep then there's little harm in theory and largely in practice. Though depending on the ad and content it may have the potential to back fire with some sarcastic teens.
Couple of issues with facebook to be aware of in terms of marketing. It is hugely popular and very busy, but who are the ones clicking on the ads and what products are they clicking on and why?
There is a distinct split between what your market may use facebook for and what most of those who click on the ads use facebook for. Among an older population it is still a "social networking" means as in keeping in touch and saying howdy. To a younger population it is a mini-internet where you will click on just about anything that crops up anywhere near you. However, is this your market?
Second, some companies have got huge responses from ads, but then who are they? Generally known brands of certain consumer items that also offer a chance of winning a years supply of something like Giant Chocolate Buttons. If you're offering mortgage advice as an example, is that going to appeal? Though if you offer me a chance to win a year's supply of Giant Buttons, I'd be there in a heartbeat.
Another issue is whether you're interested in personal or company business. Most employers block access to social networking sites so people won't see it during working hours, are they likely to remember it and access it during working hours?
However, some of the demographic assumptions are just that. Most of the more mature users claim they just use it for "keeping in touch" but they're just as much addicts for pretending to be farmers, thinking I give a damn they think they're smart because they did ok at scrabble or some form of special gun I can blow people apart with on mob wars.
The conclusion here is that there is a reasonable spread of different ages using the site, but is it enough for the time, trouble and effort required?