That's from Jane the Actuary, who actually did start her own blog, Jane the Actuary. If you go over there and read it, will it save her from the fate of writing for "imaginary readers"? She'll still have to imagine you to write for you. From the beginning on this blog, I wrote for myself. I wrote about what I was reading and I wrote to see what I thought. It was a way of thinking. Since Meade moved in, I also write for him, often saying "New post alert" and then hearing what he thinks, often as he reads it aloud (and helps me find typos and hard-to-grasp phrases).
That reminds me of something Vladimir Nabokov said in 1966, in answer to the question "What is your greatest pleasure in writing?"
There is the first satisfaction of arranging it on a bit of paper; after many, many false tries... The next pleasure is of reading it to your wife. She and I are my best audience, you see. I should say my main audience. After that, when the thing is published, I do imagine a number of people whom I like, whom I admire, with whom I feel close kinship. It is nice to think that those people are reading that, perhaps at this very instant. But that's about all. I don't care about the general public.