To expand on what @luke said implicitly, there exists digital "levers" at these sites which control - amongst other things - the probability of a post appearing on your timeline (although they might not describe these as such).

Since your timelines are not chronological (unlike the Fediverse), Twitter/Facebook can increase the probability and "nudge" a post to your timeline.

This also means that a company can lower the probability of a post appearing, unbeknownst to a poster or their followers. This is "shadowbanning".

Nudging and shadowbanning are two sides of the same coin. Coupled with the snowball effect, and with the metadata of every user, these mechanisms are extremely effective at changing the *perception* of public attitudes without suspicion towards the company.