There is no real secret about success over social media, the best guess is have good staff members who can spread the word, good managers, good quality and quantity of uploads/streams and be entertaining.
I've staffed many youtubers in the past up to at least 60k subscribers - these where Call of Duty players however and unrelated to the Runescape Community. What type of Youtube/Stream you trying to start up? - I wouldn't mind a new community channel popping up as the only good one out there now is Sparc Mac as Chris Archie has went dead.