Mayor Bloomberg joked about his own adventures on social media while introducing Facebook COO Sheryl Sandberg at a party for her book “Lean In.” Bloomberg pulled up what looked like his own Facebook page on a giant screen. A spy said, “Among Mike’s posts were ‘Hosting a book party for Sheryl Sandberg #iamsocool.’ Under his notifications, his friend request to Mark Zuckerberg is ‘still pending.’ And everyone got a good laugh when a post from Jay-Z popped up with a picture of Beyoncé and Mike carrying Blue Ivy. Caption: ‘Family back in NYC-me-BlueIvy-Beyonce-babysitter.’ ” In the crowd were Charlie Rose, George Lucas, Sen. Chuck Schumer, Christine Quinn and Barbara Walters.