Although I dont use twitter, I can fully understand why its become a target.
Obviously it doesnt excuse any lapse of security, but any company that has a huge amount of users in their database is going to become a prime target. The fact that they have in excess of
500 Million users means that theres a good chance that a decent portion of them will have easily crackable passwords ensuing that a hack attempt will reap good results for hackers.
Its been the same throughout computing history ie why Windows has more exploit attempts than linux, why Firefox used to be said to be safer than IE, but once FF got the majority then hackers targeted FF for exploits too. Hackers attack the most popular software (and hardware) because they know the more users, then the better the reward.
