Not long ago I installed the Tango app on my Nexus 4. I mainly did this as friends were using the app and told me I could use it for video chatting, and it seems to work reasonably well for this.
But then I found something that concerned me a little, it seems you can see your friends friends and there is no way to disable this.
To see a particular friends friends, all you have to do it block all the other friends (which may be difficult as Tango lists all contacts, not just Tango friends) then go to "friends" -> "People you may know" and you will see a list of that friends friends.
While I have nothing to hide and don't really care if other people see who I am friends with, the fact that this can not be turned off seems like something that would concern some people.

EDIT - 12 Nov 2014
It seems Tango has been updated and this no longer applies or you can now disable it.