They don't even need it to be a significant amount of their income. But they could show that some people are in fact willing to pay, which may be enough to entice big publishers to supply content to Facebook again, closing the loop on the original idea of "outsourcing [news] distribution".
But maybe, I say, it never took off for a reason? Some things are just not what people want.
(Though of course all the content providers crave them like crazy)
No one (practically speaking) accepts micropayments because there's no unified system with good UX, and no one can build a unified system with a good UX because no one is accepting micropayments. Facebook could change all that in the same way that MS can push a feature to Windows and instantly make a new market. Barrier to entry looks different when you're a de facto monopoly.
That's because there is no FB-level money to be made in "microtransactions in exchange for content", especially news content, and nobody cares for those firms, including Medium which has posted its second "We've failed to make any revenue, but pivoting to some vague BS" post by now.