I wouldn't underestimate facebook for b2b. The users that see the ads still work at businesses. I have had very good success using these for my b2b application.
I used the "likes" targeting in the facebook ads demographics to target occupations that my product is aimed at. I also targeted relevant english speaking countries.
I have tried quite a few different styles of ads, but individual ads for each occupation that were specifically targeted has the best results. I guess I would advise creating many ads, and wording them specifically for each demographic you want to target.
I also tried various pictures, but ultimately a screenshot of my app was the most successful.
The majority of my users so far were acquired via the facebook advertising, it has been my most successful strategy.
