Michel de Montaigne
According to The Writer's Almanac (28 feb 08) the art of essay writing was born in the 16th century through a need to write letters but a lack of people to write to; instead of writing to one person Michel de Montaigne wrote to a public audience.
For me this resonates with the popularity of blogs; we write blogs to an (at least partly) unknown audience because we need to write/reflect but either lack a critical person to write to or wish to write to a larger audience. In the same way once essay writing became an established form essayists wrote essays replying to, and commenting on, other essays. The "conversation" between bloggers is perhaps today's speeded up version of those leisurly times and habits!
From The Writer's Almanac (28 feb 08)
Michel went off to college and became a lawyer. His father died when Michel was 38 years old, and so he retired to the family estate and took over managing the property. More than anything, he loved to write letters, but after a few years in retirement, his best friend died and he suddenly had no one to write to. So he started writing letters to an imaginary reader, and those letters became an entirely new literary genre: the essay.