Your growth engine is very dependent on your economic engine.  If you have relatively limited revenue per user, you’ll need to pursue tactics with a very low marginal cost such as PR, SEO or viral Marketing Events.  With a higher revenue per user, you’ll also be able to effectively arbitrage growth through paid tactics like display advertising and SEM.  The most valuable companies generally choose an approach that allows them to capture the biggest share of the market in a sustainable way.  This often means a strategy with lower revenue per user.  They don’t invest too much time in one off gimmicks, instead they focus on growth drivers that can be repeatable and scalable.