We haven't yet seen Facebook attempt to strangle a competitor that it cannot buy. If Facebook faces an existential threat, there is a lot of capital available to fund its defensive campaign. Buying is just cheaper.

A direct threat to Facebook is unlikely to succeed. More likely is a competitor based in a niche where Facebook cannot enter for structural reasons that slowly out-competes Facebook on Facebook's turf.

One possible niche is the privacy-focused community. Facebook can't flip all of those users, ever. If something privacy-focused emerges that has low-maintenance utility for the broader world, Facebook's days are numbered.