Sending email to your friend is to force the recipient to read the message. It works as long as there’s a trust friendship between sender and receiver, though I’d admit that not all such messages are comfortable in my experiences.
Likewise, posting a new journal post and letting your buddy know that I have posted something is to force your buddy to read that particular post, though the degree of expectation is now less. That is, sender now expects receiver to read the message to a lessor degree at SNS journal posting environment like MIXI than it is with email environment (as a receiver, when you’d ignore or forget to read that message, it is OK). It is really up to recipient whether or not to read that message. But there is still some degrees of expectation remain. I can know when you read that message, when I know that you read that message I get happy. Even more happier, when I got comment.
In Twitter/tumblr, that expectation is now shifted one step further. you don’t know whether or not the recipient read that message and hence you don’t even bother if they read it. That is the art of messaging in twitter/tumblr, where, by default, they don’t even have the comment function.
I forgot to add blogging but it should be positioned somewhere in between twitter/tumber and SNS journal.