Need more users for my internet startup (Growthhacking) (Already some traction)

If you're just starting out, then naturally you're going to focus on getting users in the first place, and not how to retain them. That's fine.

However, quite soon after you've launched and first users join start thinking about retention. Retention is just too important to ignore. If you have a hole in your bucket, what do you do? Run faster so that less water leaks? No. You fix the hole.

Maybe below video makes clear how important retention really is:

If you already have at least some traction, use it to split test your way to success. You never know how people will react - you need to test it. Make little changes on your website, features, pricing strategy etc. Many successful internet companies attribute their success to this strategy. 

The most popular tool (because it's free) to track your users' behavior is Google Analytics, however there is also something to be said for paying a few dollars to use split-testing focused services like Optimizely or Mixpanel.