They are a private company. They can filter, block, promote any speech that they want.
I always said the same about TV and radio companies, but various Statists from FCC and FEC down to Slashdot cowards always disagreed.
Good to see some turnaround in public opinion towards liberty. Except, oh, wait, TV, radio, and even web-sites may not be able to do what they want... Even texting in support of a candidate may be illegal.
Unless, obviously, the candidate is from the Party of Government. For a few decades we had something called Fairness Doctrine, which allowed FCC to enforce "fairness". Libertarians fought it, but at least, with it on the books, one could formally complain against "unfair" coverage. Not any more — with only 7% of journalists being Republicans, the game is played with only one set of goal-posts...