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1.
Multiple profile disorder (MPD) is a social networking disorder characterized by having at least one "alter" personality that controls behavior. The "alters" are said to occur spontaneously and involuntarily, and function more or less independently of each other.

They may be evident by posts that are not characteristic of the known person and seem widely out of normal character. Some of the posts may be offensive and unfiltered.

The unity of consciousness, by which we identify our selves, is said to be absent in MPD. Another symptom of MPD is spontaneous posts in conjuction with amnesia, which can't be explained by ordinary forgetfulness.
Multiple Profile Disorder posts may include "Dear bitch who stole my parking space after I drove around and waited 20 minutes: You'll get your Christmas cheer in the form of karma."
by Sweet girl with angst issues December 06, 2010
 
2.
When a person has multiple online profiles, and at a certain point starts confusing them. For example, mixing up usernames and passwords for all the various accounts you have, or forgetting what piece of content, or witty remark, you have posted where. Someone who suffers from multiple profile disorder will mutter and swear at themselves while staring blankly at a log in screen, desperately trying to remember what they're doing at this specific web site.
Joe can't log into his Simple account. He suffers from multiple profile disorder and forget his username and password.
by Rhino Von Steiner November 13, 2013