In George Orwell's dystopian novel Nineteen Eighty-Four, a memory hole was a system of pipes for conveying documents to a furnace to be destroyed. It was chiefly used to destroy old versions of newspapers to hide the fact that the government had rewritten history. Thus, in modern parlance, a memory hole is any means for eliminating inconvenient historical records.
The term is also used as a transitive verb, as in "I memory-holed my Facebook profile". In the internet age, attempts at erasing the history of various websites and wikis, usually for the purpose of deleting embarrassing edits, are common. The various social networking sites (Facebook, etc.) have made it very easy for people to say things that they regret later. This is especially prevalent among people in the teen or "twenty something" age groups, who later find that they have harmed their chances of getting into college or getting a good job. What people often don't realize is that "The internet is forever." There are websites ("Wayback machine", for example), devoted to keeping historical records of websites, and individuals also keep copies of things that they think may be memory-holed.