I was browsing reddit mindlessly when I saw this maybe ~10 posts down from the top of r/all, and I kind of snapped:

I am not a communist. I think violent communist revolt would result in, at the very least, a period of significant loss of life and economic destruction. Successful communist revolts usually (but not always) produce brutal, authoritarian police states.
Yet it it seems to me like violent communist uprisings are fairly overtly supported by a sizable fraction of the American populace, perhaps 1-5%. When I talk to the larger fraction of the American populace that does not actively sympathize, they appear to be much, much more comfortable with this than I am. They speak as if the FBI was somehow omnipotent, or as if the internet has removed the possibility for popular revolt inside countries with strong civil liberties.
This optimism is insane. Authoritarian factions are adversaries. They are run by real people with real political ambitions who attempt to move around what you think are the tripwires. They will mask themselves even to internal supporters as "social democracy" or "alternative right" or what-ever, and take off the mask right as they are lining you up against a wall. When they do, our turtle-paced glorified three-letter bureaucracies stand a real and substantial chance of being too slow to react.
A large part of this procedure is the revolutionary alerting everyone of their pending revolution before anything actually illegal happens. Statements like the above accomplish a task in this vein: demonstrate widespread sympathy and tolerance for murder. They serve as an important coordination mechanism, and that is the job they are performing in the above post.
In the general case I think most people intuitively understand why this should make everyone nervous. If 1-5% of the United States were upvoting posts on Reddit explaining how they "unironically" support a fascist uprising and gassing all Jewish Foreign Agents, well... finish the sentence. I would at least not have to explain why this political propagandist threatening to kill a large number of people is a bad omen and why it's a cause for concern.
So I'm asking: what can I do now to help prevent this from happening? I don't want to wait ten years so the problem can get 10-% worse. I don't want to deflect by saying that the meteor has got at least 30 more years on its flight path before it kills 25% of the people in my hometown. I want to act now.
As a starting point, you can open the tweet and report it to twitter. Twitter will sometimes delete the accounts of users who threaten violence if it is reported to them.
On a deeper level, the problem is that freedom/liberalism is inherently self-defeating. Any form of freedom involves giving people the right to accumulate power; but once they accumulate enough power, they can use this power to take away other's freedoms.
In this case, the "power" is organized social networks, and the freedom in question is freedom of association. In order to stop communists from organizing a revolt, you must dissolve their social networks, e.g. get forums like Twitter to ban them when they threaten violence, but doing so violates their freedom of association.
If you're just trying to protect the economic conditions as they exist now, then that might not be a problem, but you explicitly say that you want to protect libertarian democracy, which runs into the paradox. I'm not aware of any full solutions to the paradox; having rules like "don't threaten violence" helps I guess, but they tend to be justified using lies such as mistake theory, that "you should spread your policies through debate, as participants are honestly trying to improve the world for everyone", which seems like a bad foundation.
I just did report the tweet. Following their questionnaire for the report, it's a specific group of people being harassed or threatened with violence. This is being done by wishing harm on people, though their identity isn't targeted. Straightforward violation of Twitter's content policies.