Now an audience of more than 1 billion people is only a click away from every voice online, and remarkable stories and content can gain flash audiences as people share via social networks, blogs and e-mail. This radically equalizes the power relationship between, say, a blogger and a multibillion dollar corporation.
The relationship between WordPress and Tumblr has always been pretty friendly: Tumblr's own blog used to be on WP, WordPress.com supports Tumblr as a Publicize option alongside Twitter and Facebook, our Akismet team sends them daily emails of splogs on the service, and there's healthy import and export traffic both ways.
I don't think BuddyPress will be something you use instead of your existing social networks... but if you wanted to start something new maybe with more control, friendlier terms of service, or just something customized and tweaked to fit exactly into your existing site, then BuddyPress is a great framework to use.