Can you use Telegram to talk to friends on Whatsapp or Messenger?
We're in a period of closed chat ecosystems (Joe Schmoe doesn't use Matrix, IRC or XMPP) and fairly spacious smartphone storage, which in my circles means that people tend to be quite willing to install an extra app for chatting.
A bunch of my friends use Signal, and the ones that don't I communicate with on WhatsApp.
Chances are you'll see a number of your friends have it installed already as others have noted in this thread.
Then talk about it to your friends, show them the stuff you know they will love. For some it is bots. For others it is themes. Hashtags to follow topics. The possibility to have a chat with yourself (I run a journal like thingy where I note when I check oil on my car, when I changed my contacts etc, all hashtagged so I can retrieve it by tag later.)
I think at some point they had the (IMO possibly good) idea that they would make the client free of charge and make money by charging for API access for businesses etc but that was before (IMO as usual) they decided to go all in on scooping up user data and trying to copy Snapchat again ;-)
Either I am a very good salesman or it isn't really hard.
I have participated in getting hundreds of people to ditch Whatsapp for Telegram (family + a few local groups I am member of).
I just started by using it to communicated woth my wife, then with close friends, then recommending it as enthusiastically as I used to recommend Whatsapp.
When "everyone" find they are using Telegram foreverything but a few groups then they will start bugging the rest of the members and suddenly even big groups switch.