Hi Jim
Well I've been 'using' Tumblr for about a week now - at first it seems it's another of those gimmicky sites but horses for courses - there are a few observations i've made...
I've spoken with a couple of friends who use it principally to share photo's with family and friends a sort of Twitter for photo and video.
It has a very useful function that lets you look back, diary style, at your previous history so you get a representation of either a photo, a headline or a video you shared.
Some may well use it as an online scrapbook - a snippet of news, music, image or video as they come across it - there's a handy 'Share on Tumblr' gizmo that you just click on and the medium you're looking at is shared instantly to your Tumblr account.
From a networking / marketing commercial use, It seems that Google does crawl Tumblr and present search results accordingly so as another means of incoming links, then it would seem it has its uses???
That's all for now. In conclusion, Facebook is about who follows you and what you can share, BC is about the discussions and following individuals or 'groups', Twitter is about sharing short 'this is what i'm doing now' type messages with URL's and Tumblr has it's own niche of an online scrapbook.
A bit messy to wade through someone elses, but you might pick up a headline, a image or a video that appeals to you.