@Doug_Bostrom
As stated, the question has a well established answer: with good moderation. Details of what constitutes "good moderation" get hairy very quickly, but the same is true of health care or construction, but those aren't areas seen as being a single problem.
The commercial social networks have the problem that hiring moderators costs money and establishing norms takes time and community deliberation, which again costs money. Meanwhile they make money by distributing largely unwanted speech, making for a stupendous conflict of interest with regards to moderation.