All alternatives, like Signal, are getting a huge unexpected influx of new users from Parler's shutdown. One of the others was having similar problems a day or two ago. I think I remember seeing that Signal had the biggest boost, too.
Parler was pretty much the antithesis of end to end encrypted and secure, and Signal serves a very different purpose to Parler. From what I've seen on Parler, their average user would dismiss Signal as being produced by commies and therefore evil, due to its Open Source license.