Email as a protocol explicitly does not come with any guarantees (of privacy, or sender's identity, or receiver's identity, or anything else as far as I can tell).

Facebook makes a lot of noise about privacy (using their own definition, naturally, being 'preventing anyone outside Facebook from seeing your data') so it's reasonable for a nontechnical user to not expect that an acquaintance installing a "what's your pirate name arrr" app should lead to their 'share with friends only' info being acquired and sold by a data broker.