I think the key is to just make the best service. For the masses, privacy and security aren’t selling points. Make the core product great, and then it happens to also be privacy respecting FOSS.
Of all the FOSS software that has made inroads with the masses, none of it has been due to it being FOSS or the privacy. People don’t use VLC because it’s FOSS, they use it because a video file didn’t play in the player they had, and VLC played it without and fuss.
Solve problems users have. They don’t see not being FOSS as a problems. So yes, make FOSS, but don’t expect that to be your headline feature. Like it or not, very few people actually care.