Mary-Kate and Ashley Olsen were out and about in the Big Apple yesterday after joining fellow stylish celebrities at the Met Gala Monday night. The star-studded ball was one of several high-profile events the sisters have attended in recent weeks, having also appeared at the opening soiree for the Tribeca Film Festival and a charity fundraiser for Women in Need. The fashion moguls have a lot to celebrate lately, gracing the current cover of Newsweek for a feature that touts them as "America's Next Billionaires." The title is well-deserved, as the Olsens' fashion empire only continues to expand. So far in 2011, they've added more luxurious pieces and materials to their collections for The Row, launched an optical division, and will join the world of online shopping with their upcoming venture StyleMint.