We just tried a small campaign on behalf of a client. They have some very good, albeit small interactions online. They sell a hardware/software networking device and they usually converse with Micosoft MVP types, IT Consultants - that kind of area. We've had some good wins on twitter - like securing support for an IT award in the UK based on user voting from a link from a tweet.
However, the PPC campaign was relatively expensive - about €2.20 average CPC and the campaign traffic was drowned out by much higher and more consistent organic visits to the site. Also, you need to be careful about tracking. We found it very expensive comparatively.
We're going to try a Follower campaign next to see that, to see can we raise followers.