The idea I came across was actually going to people on the street and show them the app, start asking them questions and hopefully sign them up.
So I went and did it. At the time of the launch I happened to be in Berlin, Germany. It turned out to be a great place to test this idea, since the people you run into are friendly and attentive.
After a couple of days, I understood the true benefit of it. At first my goal was to get people to sign up, so that my app will have some activity and users, before I show it to the internet world. Although I did accomplish this goal, I also gained something much more valuable.
The feedback I received about my app from face to face encounters with strangers was incredibly useful. People spoke very candidly about what they liked and what they did not like. For example, they all liked the questions and thought they were interesting, and disliked the registration process (which I later dumped). I saw on their faces the reaction to different parts of the app, and learned when and how they understood what they were supposed to do in each part. This greatly helped me improve the usability and clarity of the site.
Before launching to the internet, I already changed and improved the site based on the feedback I got from random people on the street. No doubt, the site would have looked much different if I hadn't done it, and it might have been less successful.
The important point is that often web developers don't consider testing their app on real users they meet face to face, in order to get feedback and appreciate engagement. I think it is fundamentally wrong. A lot of usability analytics and research can be done much more effectively face to face. It only involves going for some fresh air and talking to people!