* Yazidi women were traded on social media, say activists * Report accuses Big Tech of facilitating IS trafficking * Governments urged to probe platforms' content moderation By Maya Gebeily BEIRUT, Feb 17 (Thomson Reuters Foundation) - Wahhab Hassoo's family had to pay $80,000 to buy the release of his niece from the Islamic State (IS) militants who abducted her in 2014, and then offered her "for sale" in a WhatsApp group.