Follow https://www.reddit.com/r/syriancivilwar, look at some of the links from Twitter and click around a bit and you'll find plenty of IS sympathisers.
However, Twitter is fairly active in shutting them down nowdays. What most do is keep a Tumblr with a link to their Twitter account, and link their Tumblr from Twitter. Then when one shuts down they can reestablish from the other.
I'd be surprised if IS aren't using Telegram though. They are pretty active on every other social platform.