Myspace was a huge success because it was the first. It had and still has so many flaws that will make it fail without major change.
Friendster - appeals to Asians I believe because it's made for Asians.
Facebook - Built for Harvard students, then college students in general, then it changed it's image to full scale. It dominated the market because it was mostly made up of college students and not a ton of spam at the time.
Every socialing website will fail over time until one emerges that is amazing at reading the market and what people want in a socialing website, but that's hard because the internet is still young.
Never heard of tagged. Probably don't want to either. Facebook is good enough for me right now.