Hundreds of twitter accounts hacked , and the hacked accounts are sending twits like “Check out this diet I tried, it works!”, or “I lost 20 lbs in 2 weeks” . This may be from your friends but don't click on such links.

If you think you've been hacked, go here: http://help.twitter.com/entries/31796-my-account-is-compromised-hacked also inform your followers not to click on the links (if you can inform them)

You can read more here

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