This past weekend I was looking to add a document database to a side project and I never considered using MongoDB. Maybe it has improved over the past few years, but when I last played with it I found it flaky and unreliable. I eventually found RethinkDB and think I'm going to go with that.
That said, MongoDB is still plenty popular [1]. It's just no longer the "hot new thing" that gets 5–10 posts a day on HN.