Echo chamber probably isn't the best framing for it. I would say that the ethos of FOSS and the values of privacy which might incline a person towards these platforms is generally more in line with liberal views than conservative ones. What I've seen personally however, is that it's not about whether you're right or left, but in how you articulate your ideas. Low-effort posts that go against the grain, may very well be downvoted, whereas a well thought post that runs against that same grain might be met with discussion, but not necessarily downvotes.