Over time, Facebook groups and Twitter have become the de facto discussion hubs for communities that I've been a part of for years. Those platforms make it hard to not reveal your true identity online, and you have to give Facebook and Twitter your phone number just to register. Facebook will even ask users to verify their identities with their driver's licenses. At least for me, this causes a chilling effect on my online participation in these communities.
For example, I won't talk about politics in my neighborhood's Facebook group because I don't want to be harassed. I recently witnessed a single mother get harassed because she respectfully disagreed with someone's politics in the group. People started posting pictures of her kids, putting them into crude memes, and the mayor went through public records to find her full name and address.
I am aware of two open source ones: RedReader and Slide (both Android).