The Trade Desk's stock has tumbled 65% in five months. Here's why it's still a compelling long-term investment.I'm quite convinced that digital advertising expert The Trade Desk (TTD 2.96%) will beat the market.I can't tell you exactly when, or by how much -- but this stock is surely heading for brighter days. It's not going to be an overnight success story, and The Trade Desk might keep lagging behind the broader market for a while. If so, I might come back and recommend buying it again, at even lower pric ...