I agree with Rob, FB mobile ads are literally the only thing that works on a pay-for-install basis in an affordable way. But i would say its essential to get your retention levels figured out as soon as possible because driving users, particularly paid ones, is a big waste if you cant make a return on your spend. For example, if your app makes money on IAP's then you need to figure out LTV ASAP or any users you get at $2 or $0.50 make not a bit of difference. That said, it takes users to get this data so yes, pay to get them or whatever u need to do. Of course, you'll be way better off if your new users are good (e.g. not random Tapjoy huslers).