You seem to have found the problem by yourself. IMO this would be the trough of sorrow[0] for any startup with advertising as a monetization plan - the growth but no growth phase.
In my blog post I mentioned that here there are basically two paths to go - both of which you have identified. If you want to stick with advertising, choose one?
[0]http://www.avc.com/a_vc/2012/03/the-startup-curve.html