Facebook has become more ingrained than ever in peoples' everyday lives, but some may still have reasons to leave the social media website, whether temporarily or for good. If someone is being stalked online, for example, she may feel that having a Facebook profile facilitates this behavior. If you believe a friend of yours has left Facebook, make sure something else isn't behind your inability to access her profile.
Navigating to the Profile
Sometimes, your inability to access a user's profile arises from errors you make in navigating to it, and not because the user has left Facebook. There are several means of navigating to someone's profile. Perhaps the most common is typing his full name into the "Search" field at the top of Facebook. If you type it in incorrectly, you may receive a "Page Not Found" error message. Surer ways of navigating to someone's profile are clicking his name when it appears in your Facebook "News Feed" or typing in his profile URL in the format of "facebook.com/username."
Blocked vs. Deleted
If you legitimately can't access someone's profile, it's possible that he may have blocked you specifically, rather than left Facebook for good. The easiest way to verify this is to click the "Chat" window and send a message to a mutual friend of yours. Ask her if she can see your friend's profile. If she can, you -- for one reason or another -- have been blocked. If not, then your friend has left Facebook.
Another potential cause of an inaccessible Facebook profile is that the site is undergoing temporary maintenance. The error messages you receive when you try to access an unavailable profile don't always reflect this, so if you can't access a friend's profile, try reloading an hour or more later and seeing if his profile comes up as available. If it does, you know his profile wasn't deleted, but was rather inaccessible on a temporary basis.