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.
Yes. Those are all meaningful to native Anglophones. Those who learned English as a secondary language might have some trouble, though.
You could think of English as a contest between other languages, to see which one can get the biggest share of the etymology. French is one of the leaders. A lot of the core words come from Norman, but modern terms, particularly those drawn from cuisine and fashion, have made it in as well.
I speak Spanish (native) and Italian (fluent) on top of English (best language).
I found French class hard growing up. Until I realized that this "latin" language had as much in common with English as it did my two romanance language.
Who are "they" in your sentence ? If "they" are Facebook, then it's not the point. If "they" are the user, then I doubt that the current outrage is anything more than the scandal "du jour". I spent 10 years talking about this with people around me. Very little cared enough to even start thinking about it for a minute. Not to mention taking life decision with it in mind.
The problem is that very few people cared.
So even if people start leaving facebook in mass, it will just start all over again with another bad actor.