I am a relatively new member of Entrecard (registered about two weeks ago), but I have already seen this principle at work both as a webmaster and as a visitor/card dropper. As a visitor, there have been many sites that I have dropped cards on but did nothing else with for the first few times. But on the sixth or seventh visit, I noticed that the site had published an interesting article, so I stopped to read it and saw an opportunity to comment. There have even been a few sites that I purchased text link advertising from (usually through the ScratchBack widget) after repeat visits. So I know from personal experience that Entrecard is not completely useless.

On the webmaster side, Entrecard has been helping also. The most noticeable effect after the first few days has been the increase in Alexa rank, but I have also picked up a few comments from some of the Entrecard droppers. If I can sustain my current activity, I may be able to pick up some signups or affiliate sales from the repeat visitors. Eventually if I add an opt-in form to the site, I may be able to use the increased traffic to generate some free leads and build up a potentially profitable mailing list.