Those of you that have a tumblr: Why do you use tumblr?
The way I look at tumblr is that it's a tool for discovering cool stuff. I can just go on there and search, and roaming through blogs and repost chains I'll find something interesting in a way that's often better for happening on the unexpected than a search engine or milling around on here. Once I've found something cool I'll end up following blogs that have posted or reblogged them. I really enjoy those wow moments when I find something awesome that I never knew about, and the ooh moments when you find something new from someone whose work I already love.
This leads to a trio of things:
The first thing is that generally, I won't reblog posts that don't have a source. A single image, however fantastic it is, is not that much use to me. If an image is great, I want to know more about the people that made it happen. If you don't give the source then my interest in that post is at an end. Sometimes, I'll be sufficiently interested to dig it out myself with google's similar image search... If I find it on another tumblr that gives better attribution than you, guess who I'll reblog?
Obviously there are exceptions to that: If it's clear that the content was brought onto tumblr by it's creator, then that's fine. If the source is really obvious, like it's a movie still, then that's fine too, although I'm generally looking for less obvious stuff.
The second thing is the people whose blogs I'm most interested in following. They will either be creators whose blog has a 'clean signal', or people who mainly reblog but focus their blogs on posting interesting content with proper attribution. I try to be the second of these, as my creative motivations fall outside what I'd want to post on my current tumblr, and for that matter my current dA account.
The third thing is that term I just threw into point two there: clean signal. (Bad form to use a term before I've said what I mean by it I know.) If I find an awesome photo you took or costume you made or picture you drew, I'm going to be straight on the source blog to see if I want to follow you. If your blog is really focused on what you do, and you're posting your own stuff and reblogging related work that inspires you, I will probably follow you, even if it looks like your posts are weeks apart. Your blog has a clean signal and you're awesome.
If your blog has dozens of reblogs of silly cat pictures and image macros and random GIFs on every single day that falls in between your creative posts, I just can't afford to have it on my dash. Your blog has a noisy signal and if I follow it, it will drown out not just your own creative posts, but those of everyone else I follow. You're probably still awesome, you're just not very tidy about it
Anyway, that was my tumblr ramble, and let's be clear, it was a ramble not a rant. You should use tumblr how you like. This is just how I use mine. I'm not saying I'm perfect and that this is the best way of doing it. At the moment, I'm even pondering splitting my tumblr up into side blogs, because although I think my signal is pretty clean, it's a really odd mix of stuff!