Facebook is by far the best social networking site out there. It keeps you connected with friends family and distant acquaintances. It has a plethora of applications and games that you can share with all of your friends. It let's you know what your friends are up to, in fact the only reason I remember most of my friends birthdays is because Facebook reminds me.
If Facebook did begin to charge I would probably pay a small amount of money for it. The real factor that plays a role in my willingness to pay would be how many of my friends stayed on the site. If all of my friends left the site there would be no reason for me to stay on. Assuming all of my friends stayed on i'd probably pay about 10 dollars a month.