Share is to Facebook as retweet is to Twitter. It’s all about how VIRAL you can get your fan page wall posts. Of course, you want plenty comments and likes on each post — this activity certainly helps boost your post visibility in the news feed of your fans. But, if you can craft the majority of your wall posts in a way that inspires your fans to say:
“Wow, I just *have* to share this with my friends!”
…then you’ll begin to notice a nice uptick in shares. Shares have more weight in the news feed. Basically, Facebook’s algorithms determine that the more people share a post and make it go viral, the more people want to see it. Makes sense.