I fail to see how this is the case. It does absolutely grant the power to regulate speech. What Social media platforms have changes is that the people who are carrying out the informal regulation of speech have shifted to being a very narrow subset of the population, and one that is overwhelmingly made up of one category of culture. Tech, especially in the Bay Area, is effectively a political monoculture. Support for Republicans is often in the single digits[0].
The evidence really does indicate that sunlight is the best disinfectant. Despite the constant concern over Trump's comments on immigrants, for example, support for immigration in the US is at record levels.[1][2] Despite the concern over explicitly fascist rallies at Charlottesville and DC, these rallies actually cased a significant drop in support for far-right.[3] And lastly, while the Republican has become a party of Trump and has adopted much of his rhetoric the result has been largely bad for the party. They lost over a dozen seats in the midterm election - surprising given that the midterms are when the Republicans tend to do well.
Yes, sunlight is the best disinfectant, and the data demonstrates it. Ironic, then, that some would want to shield these views that they despise from said disinfectant.
0. https://www.recode.net/2018/10/31/18039528/tech-employees-po...
1. https://www.nytimes.com/2018/06/23/us/immigration-polls-dona...
2. https://news.gallup.com/poll/235793/record-high-americans-sa...
3. https://www.aljazeera.com/indepth/features/2017/09/alt-weake...