With StartHQ (http://starthq.com) I've been posting comments on HN, as well as relevant Facebook groups like local JS meetups, startup incubator & meetup groups etc. Those seem to work much better than e.g. LinkedIn or Google Plus.

One thing that dramatically improved the conversion rate was a blog post giving the background on what problem the service solves: http://www.arcticstartup.com/2013/03/11/starthq-targets-and-...