Hackers might have accessed and posted the information of up to 4 million AdultFriendFinder users.
Users' sexual preference, marital status, email addresses, dates of birth and physical addresses, might have been stolen.
The site's parent company confirmed to USA Today that it is working with law enforcement and has employed FireEye's Mandiant to investigate the possible activity.
"FriendFinder Networks Inc. has only just been made aware of this potential issue and understands and fully appreciates the seriousness of the issue," the company said.
FriendFinder Networks Inc. owns AdultFriendFinder as part of its 40,000 affiliated websites. More than 600 million registrants are in Friend Finder Networks' system, with AdultFriendFinder accounting for 64 million members, the company wrote on its homepage.