How is social security currently funded?

What happens to social security funds once they are paid into the system?

What changes have been made to the social security system from inception to date? Are those changes adequate?

When would the US expect the Social Security system to begin to experience difficulties, if any? Why?

What are the pros and cons of replacing social security with private funds? Is it good to support privatization or not support it? Please provide data/arguments to support side of privatization or not support it.