The Importance of Being Earnest
Act 2 Scene 2 :

Cecily. I think your frankness does you great credit, Ernest. If you will allow me, I will copy your remarks into my diary. [Goes over to table and begins writing in diary.]

Algernon. Do you really keep a diary? I’d give anything to look at it. May I?

Cecily. Oh no. [Puts her hand over it.] You see, it is simply a very young girl’s record of her own thoughts and impressions, and consequently meant for publication. When it appears in volume form I hope you will order a copy.

Oscar Wilde, of course, with the difinitive reason d’etre of blogging.
(A hundred years, or more, before the first one)