Just some thing I happened to follow from their infancy, and got to witness them making it big time, and now I get to go "pfft, I knew them before they were cool".

• Imagine Dragons - I remember them posting a free song download on Facebook to celebrate 6,000 likes. You were pretty much guaranteed a personal response to your posts and comments on their Facebook page. Now they're a globally successful group, getting billions of views on their music videos.
• videogamedunkey - Was among his first 1,000 subscribers, heard about him from a friend of his who I used to watch. Now he's at 5.5M subscribers, and over 2 billion total views.
• Terraria - Was among the first players to play it in leaked alpha, the community was so small, you could chat with the developers in their IRC chat. Now it's among the highest selling games of all time.