Why is Facebook Worth Anything? (#facebook)

Apparently, some people are pricing Facebook at $33 billion.  Why?  Have you ever clicked on an ad in Facebook?  I haven't.  Do you pay Facebook any money for their services?  I haven't. Who is, and why would they?  As far as I can tell, Facebook offers free web hosting and almost nothing else.  They generate no content. All activity is user-generated.  Can someone explain this mystery to me in the comments?