1) Lines are used to divide content and guide our eyes to what is important. In this case, we have to travel "across the line" to see the next piece of content which may be related, which breaks common notions.

2) Our mind usually focuses on the largest items on a page and then goes to the smaller details afterwards (Like how magazine article titles are always bigger than the text below it). In this case, after the top photo, my eyes move towards the bigger content boxes than the thin line in the middle.

EDIT: A third one!

3) If we focus on the line, the content to the left of the line becomes slower to read than the content to the right of the line. When we look to the left of the line, our eyes have to travel across the entire box to get to the start of the status. When our eyes travel to the right, the status starts a lot closer (due to text being written from left to right)

Here's an example:

Its a great day to be a programmer | I can't believe I got this job!

If you read this left to right, its fairly to read. But if you start in the middle at the line and want to read the left comment, your eyes have to go all the way to the left to get to the beginning, then come all the way back to where they were in the middle at the line. When you read the comment to the right, the text starts immediately.