The good news is that the Rockstar Social Club, the hub for Grand Theft Auto 5's multiplayer mayhem, has not been hacked. The bad news is that another unnamed site apparently has, and you should probably change your password as a result.
Word of trouble originated on Reddit, where a number of posters reported that their accounts had been taken over by unauthorized users. Numerous complaints were also logged with the Rockstar Support Twitter account, and the whole thing had a whiff of a "larger hack going on," as the original poster put it. We also received a tip from a reader who claimed his account had been compromised, despite being only a month old and using a password unique to his Social Club account.
However, in a statement sent to Kotaku Australia, Rockstar said the Social Club has not been compromised, but did acknowledge that "attempts have been made to access user accounts using email and password combinations from an unaffiliated, compromised website or database elsewhere on the internet." It's now in the process of reverting lost accounts to their original owners, and recommended that users do not share email and password combinations across multiple sites.
It's a wise policy—and even though the site hasn't been hacked, I changed my password anyway. Probably wouldn't hurt for you to do the same. If you're experiencing issues, Rockstar's technical support number in North America is 866-922-8694.