This is revenue and not profit and I've seen a lot of articles claiming that they are almost definitely selling headsets at a cost already[0]. Point stands either way for giving them free and selling at a cost is just a lesser version of that at any rate.
0. https://www.google.com/amp/s/www.pcgamer.com/amp/oculus-will...,
Of course, the essence of your point is true: Facebook doesn't make as much per year per user on average as a headset costs.
[1] https://www.statista.com/statistics/234056/facebooks-average...