My facebook outbox messages deleted themselves?

There were several messages I sent to one person in October. I checked my outbox to read them and the first three were there but the remaining three were not. Any ideas why?|||On some sites it automatically deletes your trash and outbox after a certain amount of time. Also, it could be that if you were talking in a chain message (Like most facebook messages look) and you hit delete it could delete the send and received. You also could of deleted it by accident. :)