Recommendation engines -- also called paid discovery -- are making it easier for online publishers to target audiences who have already expressed some interest in a topic. In this How-To, we explain the three types of recommendation engines out there, list five reasons you'd want to use them, and warn you about the four possible problems subscription sites can have with them. Plus, we've created a super handy comparative table of the three major vendors in this space -- Outbrain, nRelate and Tab...