When you are writing an email to someone, you are communicating. When you are writing a blog post that may be read by anyone who might stumble upon it, you are publishing. (These are not absolute definitions, but they will do for the time being.)
Consider this: When you “publish” a blog post, it is still communication. All writing is communication. Sometimes even a humble grocery list is communication — when you write it out and give it to someone else so that they may do the actual buying. Books are communication — authors write them to communicate ideas to you, the reader. Topical blogs, recipe books, Facebook status updates, tweets are all communication in addition to being published material.
What makes the one-to-one email (or letter, or text message) different is perhaps only the fact that it is not “out there” for anyone to access. It’s personal communication.
My feelings about Facebook have largely crystallized into a kind of general mild distaste. The way Facebook is supposed to be used — if there is indeed such a thing as that — is between publishing and personal communication. But Facebook keeps trying to blur the lines even further and make the two acts into one. This is dangerous. No matter how many layers of similarity there might be between publishing and communicating, the distinction that I believe must always remain is the one between doing and sharing.
The act of sharing matter online has an element of discretion attached to it. What you choose not to share is as important as, if not more than, what you choose to share. Mindless posting of everything you find, do, or like makes for an ambiguous narrative and doesn't help your reader form an opinion about who you are. In order for your narrative to have any kind of coherence, there have to be gaps in it — empty spaces that emphasize the importance of the parts that are there.
So when a social network such as Facebook makes doing synonymous with sharing, it hurts the cause of publishing. It throws discretion out of the window by auto-sharing with your friends the music you are listening to or the article you are reading. Life is rich, but it makes no sense in its entirety. In retrospect, life is remembered as a collection of memorable moments. It makes sense when you put some effort into making sense of it by discarding a good amount of extraneous detail.
Great writers are remembered, not only for the things that they put out for the world to read, but also for the things they decided to keep to themselves. Imagine what your favorite author would look like if every hare-brained idea he ever had came out as a book.