— 06.30.14

Mashable’s Social Media Day connects dreamers and doers with new ideas in the digital space through global, IRL meetups. But it’s also a chance to do some social good—the kind that starts online and makes a difference in the world.

The challenge to the #SMDay community: help find a life-saving marrow match for patients like 8-year old Baylor Frederickson of San Francisco. Bay needs a bone marrow transplant within the next three months, but first he has to find a match. That’s where you, the #Rally4Bay and Be The Match come in.

The Be The Match Registry includes more than 11 million selfless volunteers who stand ready to be a life-saving bone marrow donor for patients fighting life-threatening blood cancers, including leukemia and lymphoma, just like Bay. Bay, whose parents are German and Japanese, has the highest chance of finding a match in someone with similar ancestry; but regardless of background, the more people who join the Registry, the more likely it is we can save his life and many others.

Bay is fighting leukemia for the second time. Today more than ever, he needs your help. So we’re asking you to show your support by showing us your #Rally4Bay selfies on Facebook, Twitter, and Instagram. Tag your friends and get them involved. And consider joining the Registry. Let’s put the power of social media to work by bringing hope to a hospital room in California.

Bay’s story and smile have won a lot of hearts and inspired potential donors to join the Registry. One of the early supporters of #Rally4Bay is Michael Lewis—Bay’s little league coach and bestselling author of The Blind Side and Moneyball. Lots of others have shown their support, too, though, from Olympian Kristi Yamaguchi to JuJu, one of Bay’s friends. Here are a few of our favorites.