English is already partly French, thanks to the Normans. You can usually drop individual French words straight in without a problem, and sometimes short idiomatic phrases. Trying to translate will just shed any idiomatic character. En masse is already idiomatically associated with groups of people, whereas "in mass" makes us think in terms of kilograms.

How many tons of people are leaving Facebook now?

As another example, if you translated tete a tete into "head to head", those actually have slightly different meanings in English. Tete a tete translates idiomatically to "one on one", a private meeting of the minds, while "head to head" is a direct--usually public--confrontation or encounter, as one might find in a sporting match.