From the Journal of Archmage Vargoth in Dalaran:
Entry 205) Most of the Alliance forces have left for Azeroth. Hopefully the carefully-compiled reports that we sent with General Turalyon will make it back to Dalaran safely.
Ner'zhul's reckless portals have torn Farahlon apart, its green hills now quickly crumbling as the Twisting Nether corrupts the earth. I have to find a passage back to the place I was before!
Farahlon was destroyed with the rest of Draenor when Ner'zhul opened all those portals, according to Vargoth's journal.