It used to be the go-to shorthand for psychiatrists and psychologists as a sort of 'crazy bitch syndrome' warning to slap on to problem patients that didn't respond to treatment. It was only included in the DSM in 1980, and was originally conceived as a disorder of the patient caught between psychosis and neurosis (hence the term 'borderline'). BPD is diagnosed disproportionately in women, and surveys of clinical practitioners have shown they perceive borderline patients as being '“dif- ficult,” “treatment resistant,” “manipulative,” “demanding,” and “attention seeking,"'(
source).
Lately, psychologists have found success in treating Borderline patients with Dialectical Behavior Therapy (DBT), since BPD sufferers seem rarely to respond well to more conventional forms of treatment like traditional psychotherapy, talk therapy, or Cognitive Behavioral Therapy (CBT). DBT is basically a spin on CBT that calls for a lighter touch on the psychologist's part, and incorporates things like mindfulness and tactics to help prevent and/or snap out of disassociation.
The source of its growing presence online through self diagnosis is that the DSM list of traits for BPD is sort of a mixed bag of more generic depression and anxiety traits when read by a layman; many people who diagnose themselves as having BPD forego the initial stages a psychologist/psychiatrist goes through in diagnosing any personality disorder, which is to establish 1. No medical/physiological causes, 2. disordered behavior is not being caused by depression/anxiety (or is non-responsive to depression/anxiety treatment) 3. disordered behavior is massively disruptive to the individual's ability to function in daily life and 4. is fairly constant throughout their adult life.
If you ignore those four prerequisites and don't know anything about how psychological diganoses are decided, almost anybody with basic depression and anxiety issues could convince themselves they meet 5 out of the 9 BPD traits in the DSM, and therefore that they have BPD. Most therapists, on the other hand, will have to have been dealing with a repeatedly suicidal, disassociative, and self-injurious patient with incredibly unstable interpersonal relationships and markedly disordered thinking (black and white thinking, self-victimization + placing blame, paranoia).
In certain cases, BPD can come across as Narcissistic Personality Disorder to other people in a relationship (romantic or familial or friendly) with the person, though BPD patients are probably less actively and consciously manipulative and/or malicious towards others than NPD sufferers, but that does sort of speak to the extreme nature of how the disorder presents, I think.