Posterous, the creator of a simple blogging service via email, has dramatically shifted its focus. The company now revolves around a new product called Posterous Spaces, which allows users to privately share photos and videos with friends or family. With Spaces, Posterous will compete less with other blogging platforms, such as Tumblr. This could be a wise move for Posterous considering that Tumblr just announced it had surpassed its 10 billionth post, and over the past year the service had grown from 7 to 28 million blogs. At one time, both microblogging services were growing at a similar pace,…