I'm thinking about a referral program, so that users could benefit by inviting the others. In my opinion referral programs can be useful.
Could you guys name some other efficient methods? Maybe someone could point to some good promotion examples here at bitcointalk.
Check out
Gleam, they have a cost-effective way for you to use a referral system and it is really handy for bounties. Just watch out for how you structure bounties in relation to this forum as there are rules against linking bounties to activity in threads.