90% of the blogs I read happen to be blogs about knitting or spinning. Cara of January One pointed out a blog post by Kathryn about the Knitting Blog Class of 2005. Kathryn went through a Bloglines report from April 2005 and made a list of the knitting blogs that were linked on the site (there were over 1700 of them!) and followed the links to see how many of them were still being actively updated and how readership had changed in the last four years. Only about 1/3 of the original list were left. I've heard of most of them and read several. (Update--I misunderstood the original post, and incorrectly thought that Margene had done all that hard work--thanks for the correction, Kathryn!)
So what makes these blogs last when the majority of them died out? I have several ideas. My blog certainly hasn't been very good lately, so these thoughts are mostly for my own benefit.
I think that the most important thing is that the author updates frequently. Probably no matter how witty and entertaining and informative the posts are or how beautiful the pictures, no one is going to regularly come back if you only post once in six months. (Ahem.) RSS feed readers do almost give you a second chance; if someone likes what they see they can just subscribe and your posts are automatically sent to them. If they had to manually come back and you never updated, you'd be simply out of luck. However, the RSS readers aren't a guarantee of readership. I know that every few months I go through the list of blogs in my reader and thin out the ones I don't remember reading lately.
A second important factor is that the author of the blog actively posts and comments on the blogs of other people. It's free advertising—often if people like your comment, they'll follow the link back to see if they like what else you're writing.
Third, the blogs that I tend to keep in my RSS reader are the ones that I have some connection with. Either I really like the quality of the author's work—the things they make or how they photograph them, or that the way they write about them is clear and clever and humorous. The blogs I read that aren't knitting blogs tend to belong to family and friends. They're things like places to update family photos and read the news about what's going on. One blog is by an artist friend from college who has the most beautiful, deep, thoughtful posts. I love reading what she has to say—I always feel enlightened and refreshed when she posts something new!
I don't yet have any library blogs in my feed reader, but I imagine that that the same kinds of principles will apply—are they well-written, with info that inspires or teaches me something new? Are they kept current, especially if they contain information about events that apply to me? Do they link to places that really are of interest to me? Does the author respond to reader feedback—can I make some sort of connection with the author—whether it be an individual or someone acting on behalf of the library?
If so, it's probably a good blog.