(1) Allow them to pursue "cultural literacy." This won't turn them into a homeschool mom's worst nightmare. It'll allow them to interact with others - regardless of age or background - with more confidence and understanding.
(2) Allow them to hang out with kids their age...and not only other homeschoolers. A lot of homeschoolers are "social" because they spend time with church groups, the Boy Scouts, the 4-H Club, etc., etc., etc. But it's important that they can spend quality time with friends in more informal settings, whether chatting over ice cream or coffee, viewing a movie, or hanging out at the beach, mall, or local amusement park.