LinkedIn has reasonably good tools for targeting based on professional experiences, but I never found the quality of the users coming from LinkedIn to be that good. One thing I noticed is that the people who tend to come from LinkedIn ads, tend to be unemployed and of lower skill levels. So LinkedIn is used a lot to find good people, but not for ad dollars.
Haven't spent enough time with Twitter's marketing tools to have an opinion one way or another on them.
While not a social network, I just keep coming back to Google Adwords, tends to produce the most most reliable results.
It is kind of hard to grow in it using automation due to how they handle things. The best I can suggest is trying to connect to influencers in your niche