Writing a blog helps you organize your thoughts, improves your writing skills, and teaches you to make good arguments (and recognize bad ones).
None of that requires having an audience. At least an audience larger than your friends and coworkers.
If you like to write, don't stop writing because it's not "where the buzz is at". There's more to life than following trends.
Please god say it ain't so. I would like to think that people who keep blogs (at least the subset thereof that also read Wired) tend to write more interesting things in them, and I can't imagine that being replaced by 140 characters (this notwithstanding: http://twitter.com/_why/statuses/881768089 )
Blogging is an internal thing first, external second.
You think stuff we are producing these days will last? I'm dubious. Paper lasts a decent amount of time, but digital media?
Even now, you'll see people post an article here from five or six years ago. It might have 100 comments on it. Five or Six commenters might already be dead -- including the article's author.
Now multiply that by 50. Most of the net will end up being written by dead people.
I blogged about this a while back -- http://www.whattofix.com/blog/archives/2008/09/the_dead_spea...
That, barring a cataclysmic implosion of civilization, the internet archives being populated now will likely be available for perusal 10,000 years from now.
As a corollary thought, an interesting exercise might be to only generate content -- movies, essays, fiction, non-fiction, whatever -- that you think people 10,000 years from now will find interesting. How does this constrain what projects to undertake?
This is another reason to study history: so you can imagine what people a thousand years ago would have liked.
Somehow I feel compelled to add an evil laugh at this point.
I think that the idea of a personal "blog" is dead. Other mediums have really taken precedence. Very few blogs really stand out to me, nowadays; my favorite ones tend to be more unique implementations, such as BigContrarian and Kottke. Wordpress blogs look old and dated in a lot of ways: they try too much to bridge the gap between the formal and the personal, and I think they lose out because of it.