Mashable is hosting the first-ever worldwide Social Media Day celebration on June 30 and you’re invited. Last week, we announced Social Media Day — a global celebration of the revolution of media becoming social. Readers responded enthusiastically by signing up to organize or attend more than 340 meetups in 74 different countries. Today, we also launched Smday.com to help showcase the social discussions around the event with featured photos tagged with #smday on Flickr (Flickr), tweets that include #smday, your Facebook comments, and, of course, the Meetup widget showcasing the events closest to your community. Our staff will be attending meetups in New York City, San Francisco, Atlanta and Austin, so come say hello (other cities may be added).