Well going further than that, on Facebook, for a lot of people the majority of their 'friends' are school friends (for younger people often their entire school year, even the years above/below), and extended family, people that they feel obligated to add (due to Facebook's pressure to have a high friend count).
These are ultimately people they wouldn't choose to follow the conversations of at all, given the choice.
For me Twitter was like starting with a clean slate. I just follow the people I want to follow, basically no 'friends', just interesting people.