It shouldn't be "FOSS evangelization."
It should be "FOSS education."
Teach people about potential alternatives. Teach people what the spyware software does: Do not judge for them using it, judge the software using them. Then talk about software that respects them.
Allow them to make their own choices on matters.
At the very least, if you're not a judgemental annoying asshole, they will ask you for advice when need to be.
Be a friend.