Of course going the direction of old school blogs and following those who comment on your own site has historically been a quick way to build a network. I’m also reminded of Colin Walker’s directory which creates a blogroll of sorts by making a list of websites that have webmentioned his own. Webrings are also an interesting possibility for topic-related community building.
Since Tumblr is unlikely to shut down immediately, those effected could easily add their personal websites to their bios to help transition their followerships to feed readers or other methods for following and reading.
Of course the important thing in the near term is to spend a moment downloading and backing up one’s content just in case.