I don't agree that because the primary supplier is a monopoly is justification for doing something as distracting and risky as designing your own chips.

So they need a second supplier? Do what the government do, and demand that there be a second source https://en.wikipedia.org/wiki/Second_source or you won't buy their product, or give money to some other company or companies for whom that is a core competency.

>> so it's critically important that one or both of them break the monopoly by becoming a fabless chip designer

You say this with great certainty, that almost got me on board, but saying something with certainty does not make it right. It's not critically important - what's the worst that can happen in the short term - they pay a bit more because of the monopoly? You make it sound like if the chip monopoly isn't broken then doom will befall them all. Piffle. If there is such great demand for these chips then there are plenty of other silicon companies who can be attracted by the smell of demand.

What would make sense is for Facebook to claim to be making their own to strengthen their negotiating position with the supplier, but with no real plan to design their own chip.