The law at issue still allows forums with 49,999,999 or fewer users to discriminate based on political viewpoint. It only says sites larger than that need to allow all political viewpoints (but only if they allow any political viewpoints, and only if they have users in Texas).
Legal: Message board with 49 million users that says "no Republicans allowed."
Legal: Message board with 51 million users that says "no politics allowed."
Legal: Message board with 51 million users (none in Texas) that says "no Libertarians allowed."
Not legal: Message board with 51 million users (some in Texas) that says "no Democrats allowed."
You would think that the judges would realize that it's in their best interest if they want this to stick to throw their political opponents a bone, but... nope ?
I think moderation gets exponentially more difficult, the larger a community is.
There's probably something in the legislative history[0] as to why that particular number was chosen, but I keep getting 404s and certificate errors when I try to click on links to the remarks.
[0] https://capitol.texas.gov/billlookup/History.aspx?LegSess=87...
In this case, these are platforms that don't target a local community specifically, so it does make some amount of sense that the number is based on the population as a whole.
One way to look at this is to consider that the 2016 popular vote difference between Trump and Clinton was 2,868,686 which means if your platform can change the votes of 3 million people then it can change the results of the biggest election (even if the bias of the electoral college were removed).
Of course that's begging the question, but it's not hard to imagine that on a site with 50 million active users, the most susceptible 10% could have their voting decision change based on the world they see and the arguments they read through social media.
This also assumes that there is an imbalance in terms of which side does more "influencing", or which sorts of arguments it is easier to convince susceptible people into believing.