[taikahessu]

Reminds me of a quote I once read of "marketing being a game of diminishing returns".

When you find a working marketing solution, it's just a matter of time when it dries out, because of competitors and overall saturation.

[giorgioz]

Yes but it's possible that the marketing channel is very profitable before drying out. Also the less a channel is known the more it will take to reach saturation. So there might be channels that are very profitable and well hidden, the problem is that by definition it might be too hard/expensive to find them.

[tomjuggler]

I can't rule out competition here, definitely part of it - cost per click has gone up a lot over the years