Note that everyone's favourite privacy-respecting app (mine too!), Signal, also does contacts-sharing, although it doesn't do friends discovery (so the server knows one's contacts, but one's contacts don't). If Open Whisper Systems wanted to be evil, though, they could do this form of analysis.
Back in March I laid out how they could use a private set intersection protocol to enable any pair of users to privately share their contacts: https://news.ycombinator.com/item?id=11289223 (I'm not posting this to shame them or something: March wasn't that long ago for developing a feature like this, and of course it's open source; I could develop it myself and submit it to them).
I think it's something they care about; they've just not found a solution they're comfortable with yet.
Well, that's the good thing about PIR — with the protocol I discussed, OWS wouldn't have access to one's list of contacts. They'd still know with whom one spoke (anonymising that is a hard problem), but at least they wouldn't know everyone one knows.
Back in March I laid out how they could use a private set intersection protocol to enable any pair of users to privately share their contacts: https://news.ycombinator.com/item?id=11289223 (I'm not posting this to shame them or something: March wasn't that long ago for developing a feature like this, and of course it's open source; I could develop it myself and submit it to them).
I think it's something they care about; they've just not found a solution they're comfortable with yet.