I guess the problem with growth hacks is that once they are known, they get overexploited and then lose all their effectiveness. Also, it would be nice to not have to sign up to an email to receive the hacks. Perhaps, have a few hacks that you list for free on the website and then a login to receive new ones.

Good luck with this. It would be interesting to see if you can use your proprietary growth hacks to make growthup into a huge site.