Users didn't sign or agree to anything just because they checked a checkbox next to a link to an ever-changing jumble of legalese to get past a screen. This isn't agreement, it's manufactured consent.
What is the difference? I am thinking if a person really actually cared they would have read the legal agreement before checking the checkbox in question and possibly consulted an attorney of their own. I am thinking most users absolutely do not care and agree out-right and immediately to all claims presented by Facebook. How is that not still agreement?
It's unrealistic to expect that users will read AND understand the TOS of every website AND all of the changes to the TOS that occur over time.