[michaelflux]

Facebook is excellent for it's targeting tools however it slightly looses it's value if your product is trying to target people based on their professional work simply because most people seem to use Facebook for personal things than professional ones.

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.