Facebook and Twitter are both right.
All or none, one or the other… the in-between is where the editorial manipulation happens, and the social network turns into something else, a less valuable and lasting presence.
Facebook’s position – what’s paid is earned – is friendlier to profit, which is right for Facebook considering its size and reach and stage of maturation.
Twitter’s direction – what’s earned is paid – is friendlier to value (i.e., network engagement and activity), which is right for Twitter in its still evolving state.
In any case, that is the signal they’re each sending.

We are what we pretend to be, so we must be careful about what we pretend to be.
― Kurt Vonnegut, Mother Night