Linkedin and Google seem to be the best avenues right now. Twitter, Pinterest, etc. do not seem like the right venue to market for us but I am open to suggestions.
Poor conversions are usually caused by a couple of factors:
1) Poor targeting or your targeting is off. This is usually the #1 cause for a campaign and/or conversions to perform badly.
You'll want to go back and do a very detailed and comprehensive profiling exercise of your ideal customer base.
Usually, I'd start here.
2) The next most common cause for poor conversions is your offer/messaging is not compelling enough to the traffic you're sending.
This goes back to doing a comprehensive customer profiling which also directs you to what would be of interest to your audience and the kind of messaging that would resonate better with them.
If you don't have the above correct, it's going to be a waste of money to add additional channels.
For B2B, Google and Linked are really the best sources of traffics, so it wouldn't be about looking at other channels I think, it would be revisiting your targeting and offer.