I don't know about that. The WC is the easiest access point to the sport for Americans, and I'm sure that there are a decent number who stick with it and get into the club side every four years. I don't think it'll be a staggering amount that Everton fans should worry about though.
Unless they're younger and really being exposed to something like this for the first time, I still doubt it happens very often. People acted the same way 4 years ago with the South African WC and I don't think the Premier League saw any sort of dramatic uptick as a result.