After having a few dozen images of mine scooped up by a tumblr user (let's call him "image_devourer")* and wanting to find them again, I started looking more carefully at the site architecture. Some notes:
Tumblr is like a blog in some respects but the archiving is limited.**
You can page back through every page in a tumblr or use an archive page (grouped by month and year).
Archives by month are not done with text but by thumbnail.
To limit bandwidth, there is a maximum number of allowable thumbnails. I count 306.
For a really heavy user like image_devourer, what happens to the posts after 306? They are effectively gone forever.
How to find, say, April archive, post 307?
You can still find it if you page back from the current post. Assuming 15 posts on a page, that would make it 20 pages back. You can find that by typing http://image_devourer.tumblr.com/page/20.
The problem is the page you are starting from constantly changes as image_devourer adds new posts.
In a couple of months post 307 will be on page 60, or 70, or 140, but who knows? You have to guess what page to type in. If you are wrong there is no way to orient yourself, so you are doomed to page forward and back through single pages of 15 posts.
Most people will give up at this point.
The pageback process is not by month--it always starts from the present post. So you can't start paging back, say, on March 31.
You could save the permalink to a post you think might "disappear" off the archive, but there is no way to page back or forward from that individual post, so it doesn't get you to a group of posts made around the same time. Posts have no archive info other than a post number, e.g. http://image_devourer.tumblr.com/post/498860497
Tumblr founder David Karp will be one the seven artist-technologist teams hooking up at the New Museum this month. On the AFC thread linked to in the last sentence, a commenter took umbrage at the suggestion that technologists had anything to learn from artists. Here is a good example--"David, I found some issues with your blog design..."
*Update: After choosing "ballsofsteel" as a hypothetical name I learned there is a tumblr blog with that name--sorry, I have changed my hypothetical.
**Update 2: New tumblr archive format: every image on a blog is now thumbnailized on a single page - woa. Most of the above rant is now irrelevant, but am not sure that an infinitely loading page is the solution for heavy users. Or maybe it provides a disincentive to be too prolific.