Widely used? No. I've found Session to be a decent alternative however it's still early development which entails some scuff and details as to how they intend for it to be financially supported long term aren't clear.
https://getsession.org/
TLDR on Session is that it's a fork of Signal (effectively same front end, key scheme, encryption scheme, etc) with a modified transport/delivery and notification system and without the phone-number-as-an-identifier caveat that signal has.
Note: Sorry for the wall of text below.
As for what that modified transport layer is, it's routing all the messaging and data hosting over Oxen (https://oxen.io/) which is a cryptocurrency that serves as a decentralised short term / small size addressable data store and an onion router for those messages/data. As much as cryptocurrency=bad in a lot of cases, here it kinda makes sense as it's just an automated digital marketplace for data hosting and bandwidth with tooling wrapped around it to support privacy and anonymity preserving tools without relying on some hopefully benevolent dictator to run it.
As for who's backing it, same group that develops the Oxen, an Australian non-profit focused on privacy tech and bearing the same name (Oxen Privacy Tech Foundation). While Oxen is pay for use (messaging and all that has an on chain cost), it looks like the foundation is covering the costs of running Session for the foreseeable future. Given the nature of the project, it should eventually be possible for users to pay their own infra costs however that doesn't seem to be implemented yet.
It's pretty easy to use.
1. Install via F-droid or download from the web.
2. Basic cryptocurrency wallet style setup where your account is based on a randomly generated "recovery seed" phrase (string of words with equal bits of randomness as the private key which can be used to rebuild the private key on a new device).
3. Then you can share your "Session ID" which is basically just your public key or you can pay for a custom username which is addressed to your public key (you can set names for contacts after adding them so the username is mostly for ease of discoverability).
4. After that it's basically just Signal but where you can make and throw away accounts at the drop of a hat.
My main complaints are
1. that it's a bit slow on delivery
2. The onion routing half of decentralised storage + routing is still being implemented for Session as the project is very much WIP at this stage.
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My takeaway is that provided it can stick around, Session has potential to shore up where Signal falls short. Give it a year or two in the oven and I might recommend it as a daily driver for messaging.
Likewise for the OPTF and their goals in general. It looks like once Session is "fully implemented" they are looking at trying to expand the approach to a discord/slack/matrix competitor as well which could be interesting. As far as I can tell they are just a bunch of privacy nerds with a little bit of a cryptocurrency lean to them but they are doing good work.