Like any card game worth the paper it's printed on, Hearthstone is an innately social experience. It's even about Warcraft characters playing cards in a tavern! And though it boasts online features to find matches, Blizzard is enticing players to get together in real-life with a special card-back.
Blizzard has opened a page for the so-called Fireside Gatherings (via Game Informer). Starting April 26, and running for a limited time, you can participate in or organize a gathering in your area and earn a card back. Blizzard notes this isn't the only way to get this card back, but it's a pretty easy way and lets you socialize in the process. You just need to play three matches in a gathering of at least three players. You can learn more about the criteria here.
The official site already has a listing of local events, mostly centered around coffee shops and college campuses. If you want to organize your own, Blizzard has provided posters and a downloadable events guide.