E2E doesn't actually matter that much in the end; it's far more interesting to advertisers (and therefore valuable to Facebook) to know who you're talking to than to know what you said. Three-letter agencies know this too - scanning encrypted traffic for keywords is going to get them a lot more chaff than wheat, but spotting that five suspected criminals/subversives/journalists (delete as appropriate to your political tastes...) are having an ongoing conversation is almost pure wheat in that it tells them to expend effort on finding out whether they're making nefarious plans or just sharing cat macros.