I think it comes down to what you're trying to achieve with your app.

If your goal (and your software) is based around lots of users, then clearly free is optimal. There are lots of studies which show the power of free, and how free is much more appealing than something costing as little as 1c. It makes sense that FB is free (or at least has a free tier). FB is useless without other people.

If you're writing an app that is entirely / mostly non-social, and your worth is not measured in the number of users you have, then paid makes sense.