Was Yahoo Answers the birthplace of internet trolling, or was it Facebook or MySpace?

Usenet. Internet trolling dates back to the early years of ARPANET and it's successor USENET. Usenet was public and anyone with a computer a phone and a modem cradle could get on.  Usenet is still out there but it's a shadow of its former self. Usenet's peak was in the 1990's and early  2000's.  I still check out the binaries for machine embroidery patterns on occasion. 

In he WWW era and  post eternal September it's definitely my space where the first trolls found a welcome home. The chans are/were too obscure for most so MySpace is where they went. 

I would say MySpace, YouTube, and Yahoo Answers, since they're all about the same age.

Part of me wants to say 4chan more so made it "mainstream", but some forms of such possibly started before that.