Lemmy

 Lemmy  is open source bookmarking and commenting software that supports ActivityPub so multiple instances can in essence provide a distributed version of Reddit, and it supports at least sending Webmentions as well.

IndieWeb Friendly

 * Webmention support, as evidenced by this Lemmy post: https://defed.lemmy.tf/post/154469 sending a Webmention to the post: The IndieWeb Movement: Owning Your Data and Being the Change You Want to See in the Web, and showing up as a mention in that post’s Interactions section.
 * There is also an issue for improving the Webmention support: https://github.com/LemmyNet/lemmy/issues/1982

Mobile Apps
There are several open-source native mobile apps available for Lemmy:


 * Mlem for iOS. (Beta.) Github, Apple TestFlight.
 * Jerboa for Android. GitHub, Google Play. See GitHub link for additional app stores.