So I looked at 45 Catholic blogs that began a decade or more ago, more or less at random (looked at blog lists on blogs), and assigned them birth and death dates. Death dates arbitrary; if someone blogged once or thrice in a year that year does not count.
Some blogs I couldn't easily track down (like Dylan's) so they weren't included.
The average lifespan is a surprisingly healthy 10 years.
(For best results, click to enlarge.)
I marked a bold line at the end of 2009 because it seems like that's when a lot of blogs fell off or began to. I suspect the rise of Facebook at that point:
I think RectaRatio.blogspot.com comes as the biggest surprise to me. It looks like a labor without ego - no writerly voice at all, just a pious commonplace blog of clips and pics, of prayers and saints - for going on 15 years! I can't recall pious blogs like that lasting too long, although theoretically it should be easier since you don't have to create content but borrow it. On the other hand I doubt these blogs get many props or hits, so there's less incentive.