The ideal usage is no more than saying "friendDA?" to signify a shift in tone and clarify implicit assumptions of the conversation. It's easier to say than "this next idea means more to me than the usual shit we talk about, can we keep it between us?"
It might be worth looking at Rands original post. http://www.randsinrepose.com/archives/2008/10/19/friendda.ht...
I don't think it's any more effective than just asking the other party to keep something quiet, other than possibly being slightly more memorable, assuming you don't get FrienDAs all the time. To me, it's mostly just a joke.
(And thus, that's why I didn't upvote this story -- who finds the FrienDA and says "finally, I can talk to people about things!"?)
2. Would your friends be douches unless they signed an agreement not to be?
(by framing a conversation to prevent honest misunderstandings that nevertheless cause feelings betrayal and violation of trust).
... and he finds friendDA 9 months later?