How is this even legal? By clicking to that site, i'm agreeing to allow FB to do whatever ONCE. How do I get out of the agreement if I find it doesn't suit me?

How is this even legal? By clicking to that site, i'm agreeing to allow FB to do whatever ONCE. How do I get out of the agreement if I find it doesn't suit me? This is like saying, "By agreeing to take this person on a date one time, you're agreeing to take this person with you wherever you go from now until whenever."