Surprisingly many, if you're bold enough to try.

The only reason me and my circle of friends use Telegram is because one of us was using an Ubuntu Phone at some point, and that was the only acceptable messaging app he could use, so we all moved on. It requires a certain authority in your group, but people are likely to adapt to you if they care about you, even if your choices inconvenience them slightly.

Of course it's never going to be "all their contacts", but the only platform that actually has all my contacts on it is good old SMS anyway.