We Are Crazy is an international organization that provides one of the most fun and innovative ways for you to volunteer online. They deal a lot with social media marketing and have interesting roles for you to choose from, either education-related or their worldwide club.
They also provide flexible work hours (you get to choose between 1 week, 3 months and 6 months; 1 hour daily for each) and a certificate at the end of your volunteering journey.