Movie stars are flocking to Twitter as Oscar season approaches, hoping to convert online buzz into box-office returns. But not everyone's got the hang of the medium, industry insiders say: Some celebs signed up but failed to start tweeting, while others tweet so often that they could actually be damaging their brand. "You've got to warn the rogue Tweeters," one publicist said. "You have to remind them: 'There are maybe 500,000 people reading your every word. You cannot assume everyone will understand your sense of humor. Irony isn't always obvious.'"