Advertisers don't care if it's a professional site or not. Look at the Oracle ads in consumer news magazines. Companies will advertise where they can reach their target market.
Right now on my Facebook page I have two professional ads (one for a CRM system and one for a business IT news site) and two Facebook game ads.
On top of that Linkedin is truly awful when it comes to targeting. Their job categories are ridiculously broad, if you want to target Software Engineers or Aero Engineers, you can't. Instead you have to target "IT" and "Engineering" - categorizations which combine together barely related roles.
It would be very easy for Facebook to offer more precise targeting, that alone would be a killer feature over Linkedin.
Search has a higher CTR but not a higher eCPA, a well designed Facebook ad can convert much more cheaply than buying targeted keywords, you'll need to buy many more ads, but the end cost is what matter.
(Incidentally most local companies don't advertise online at all, it's not a case of stealing customers from Google, etc. but rather a case of persuading them that it's worthwhile. Hence the rise of local ad brokers who buy CPM and sell CPA, as it's much easier to convince a local business to advertise through you if they don't have to pay you until after you generate business).