Why does Facebook use hash in the URL...sometimes?

Why does Facebook use hash in the URL...sometimes?

I've noticed that Facebook SOMETIMES uses a hash in the url. For example,
when I'm on my home page, and then I click my profile page, the URL
quickly changes to:
facebook.com/#_________
followed by some other stuff.
before loading to:
facebook.com/profileName
It doesn't always do this. What's going on here?