If, let's say, a group advocating for the abolishment of asset forfeiture wants to communicate their message, and the media is not covering it, it would be natural for them to try paying for ads, and the obvious place to put ads are on social media, since that's where the eyeballs are (cf, the collapse of the news media). If social media bans those ads then while the group clearly still has free speech, they will have little way to meaningfully exercise it.
You might say - and I'd certainly agree - that the real problem is a world where everyone is on Twitter and Facebook to the exclusion of all else, and thus a world where 3-4 CEOs can unilaterally decide that nobody needs to hear anything about asset forfeiture (or whatever). And yet, that's the world we're in.
(You also don't seem to understand what "free speech" is. By your logic, a law banning the purchase television ads for Democratic candidates - but allowing the purchase of television ads for Republicans - would be okay, because after all, those ads cost money, so it's not restricting the Democratic candidates right to free speech. That's absurd, no?)