You might want to start by defining the goals for this app. Some social apps have a very limited group that would be interested in them, others want to be the next FB.
The reason for this is that for social apps, they follow the "demand-creates-demand" business model. That means that people will go there only if there is a certain number of other people that go there. This is less true in very specific targets where they would expect a small group.
If you must get a large number of people on your app to have it work, then you'll have to invest in something that gets them there and keeps them there.
I signed up for a social membership because of a great deal they were offering, it was a $20 Amazon card for $10. They sold out quick, but lost a lot of the members because the offers after that weren't very good or mainstream interests.
They invested millions getting people to sign up and it didn't work great.
There are many many apps/sites trying to get people to sign up, but people don't bite unless there is a compelling reason. You have to determine how many people you need, the more you need, the better the offering has to be.
If the app is for a smaller select group and there aren't other apps for this, they you have an advantage. If it's more general, you'll have to offer something great in order to get people to have an interest in it.
Consider the web itself, most people visit a small number of pages, maybe 30, yet there are millions of pages out there trying to get attention.
Point: if is a special interest group that you are targeting, you can make a good app and win ... If it's a broad general interest group, you'll have to offer something awesome.