I think the author means that tweeting keeps you from writing a diary in a way that it makes you feel that you already wrote about your happiness, problems, emotions, etc somewhere else.
In my opinion there is a misleading feeling of privacy in social media in general. If you post something online, by keeping it &quot;private&quot; you may be mislead to think that no one else is going to be able to read&#x2F;see it at any point in time. There seems to be no space where you can currently be alone with yourself anymore.