I didn’t want to write things 50 times, but I didn’t want to leave folks out either. Or overwhelm my busier friends. With blogging, everyone can choose how closely they follow my life, or not, as the case may be. You have the power! 
From: http://blog.penelopetrunk.com/
1. Email is inefficient.
Email is one-to-one communication and social networks one-to-many communication. (Here’s a good link about that.) If you have something meaningful or thoughtful to say, why not say it to many people? It would mean that more people share ideas and more people understand your way of thinking. Also, there are so many pieces of our life that we tell at different times to different people. Why not just say it once? We all have email overload: we parse our messages into 40 one-to-one messages instead of just a single one-to-many message.