Research indicates that cultures in which relationships can be formed and dissolved relatively easily produce more disclosure of intimate information between friends, probably due to a combination of greater need to invest in each relationship and lesser harm from taking risks that alter relationships.
The study compared Japanese culture to U.S. culture, but my impression is that there has also been a significant change over time in the U.S., with internet access increasing relationship mobility, followed by an increase in self-disclosure. (It’s possible that my impression was due to my move from New England to Silicon Valley in 1994 – there’s more social mobility in Silicon Valley, but I didn’t notice much change in self-disclosure until several years later).
It seems likely that the effects of the web on relationship mobility and self-disclosure will grow larger. The trend of increasing mobility has shown few signs of slowing, and the effects on self-disclosure probably lag by at least a few years.