Flutter is becoming very popular for building apps. It is likely to have surpassed React Native, and according to Google more than 20% of new apps are created with Flutter today.
Anecdotally, I'm running a Flutter Meetup with over a thousand members in Stockholm, so I feel like I have the ear on the ground. From what I see, the use of Flutter has been picking up steadily. For instance, one of Sweden's major banks has recently moved its apps to Flutter (including its web app). To their customers, it still looks exactly the same, so moves like that aren't always visible. Also, it seems like a very large proportion of startups are using Flutter too.