I think the best use of blogging is almost like a journal for yourself more than for others. Blogs can be extraordinary ways of "talking out" what's happening in the classroom, of examining how you're approaching lesson design, dealing with issues, starting new practices. Whether it helps others or not doesn't matter (because the best blogs out there seem to me to be the ones where the blogger records observations, almost scientifically, about his experience -- we the readers get to make predictions and conclusions ourselves).
Also, my favorite blogs are catalogs of stuff -- books, movies, music, articles, photographs. Sometimes just updating once a week about what you're reading is a great way of keeping a "learning journal."