It's not uncommon. My cousin sent out a Christmas card announcing her divorce - I think it stops a lot of 1-1 conversations with people which can be quite draining when you're already pretty raw.
What is the alternative to announcing a divorce? Keeping it secret? Not using social media to communicate?
In this case she explicitly did NOT make any mention of the divorce on social media when her husband first sprung it on her, nor during the process. She wrote this piece after it had been finalized.
I guess a private announcement makes more sense to people than a public announcement, unless you wanted to make a blog post about a phenomenon related to it, which she appears to be trying to do
Well ask yourself why they post? Also did you read the article? Not only did this woman post it online she cross posted it to a littany of online services. Who, exactly, is she informing?