If you even get just 200 people to initially start to test it, that should start to give you some feedback, but, you will want to make certain that you have a good way to get feedback from hopefully at least 1/4 of the initial users.
Now, 50 people may not sound like much, but, if you can wow them, not with your application, but with your great response to any problems or some interesting new features, then some of them will begin to tell their friends about it, over various social networks.
If you find that the initial users are not talking about it, in a positive way, then that should send warnings that you have a major problem.
I would be careful not to have too many people initially, and it sounds like you have a decent number, to see how the first reactions are, just try to follow up.
If you had 2000 people testing it, then it would be hard to try to follow up with the majority of them, for example.
You shouldn't have to introduce much more, if you can't get people talking about it, then whatever you try to do is doomed, I believe.