Yes, what you heard is right. The major social networking site Facebook limits the users to add friends more than 5,000. Once you have reached the limit of 5,000 friends, you can't accept new friend requests. If you want to accept the friend request then you must unfriend some of your friends and replace your new friends with the old.
Most people, when they reach this limit of 5,000, they unfriend those who are not any more active on Facebook, and accept the new ones. So, what you heard is totally correct, there is a limit of 5,000 friends for a single profile/account.
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All I Can Do Is Be Me!