I think your giving Twitter a lot more credit than due, by quite a significant amount. It's reach and representation of the electorate is fractional.
The word control is not correct either - no more so do they control the public sector than news outlets that get significantly more reach and a wider demographic. People chose which channel they subscribe to and will be 'at the mercy' of that channels political bias. This has always been the way, with print media famously leaning one way or the other. Consolidation of these platforms becomes tricky as the influence gets much broader reach, hence the Murdoch family case studies being so interesting.
Twitter as a standalone platform has no where near the political reach or meaningful consolidation of the world of communications to be as egregiously controlling as you make out.
Agreed on your final paragraph excluding your final sentence. Twitter may be very relevent to you, but it is certainly not a monopoly. Facebook on the other hand and its ability to deliver precise messages to segments of its user base should absolutely be explored as it's influence could be 'too strong' for the limited reach it has. Hence, radical pockets of the population in their own echo chamber. Much like your Reddit remark (or someone else's).