Sounds like 16-year-old Canadian pop sensation Justin Bieber has some competition on Twitter: South Korea's Kim Heechul.There's a bit of an age difference -- Heechul is 27 -- but both are musical heartthrobs for thousands of teenage girls with rabid social network followings.Bieber (@justinbieber) may have the edge on Twitter followers -- with 5.6 million to Heechul's 245,000 -- but Oct.