I believe this is essentially what is going on right now, and I do believe that these effects are currently the biggest threat to democracy. Fringe opinions and downright stupidity are entering the "mainstream", they are being "discussed" right now on forums like reddit's
/r/unpopularopinion.
But I don't believe that censorship are good solutions. Strong moderation is needed to keep a forum interesting and worthwhile reading, but that's a practical issue. Government censorship goes way too far.
There should rather be laws that prohibit the use of certain online marketing tools in political campaigns. For example, it should be a punishable felony to use viral marketing in political campaigns that involves any kind of fake comments, replication bots, deliberately falsified news, or fake accounts not tied to real persons. Maybe these tools should also be prohibited for ordinary marketing - IANAL but I think in many countries they already are -, but they should definitely be strictly prohibited in political campaigns.
Right now, it seems that e.g. in the US almost anything goes in political campaigns as long as no foreigners are involved, and we know from the Cambridge Analytica case that some companies act like mercenaries for hire without any conscience or care for democracy. That's what needs to be addressed, not the lunatics and crackpots on forums who will always exist.