The site is a scam. They probably can’t add millions of users of their poor monitoring service at once, but they wanted people to sign away their right to sue before a class action suit had a chance to inform the people screwed of their rights. Unfortunately it seems to be working.
Regardless, even when they do finally sign you up, they can only offer you their credit report, and they are only one of three bureaus. Irregularities from identity theft and other fraud may never be reported to them. There are better ways to check your credit reports with all three bureaus without signing up for their “free” service. Follow the link in my comment above for advice on how.
The cardinal lesson is that you cannot trust the credit bureaus. They are absolutely not on your side. You aren’t their customer, you’re their product they sell to the financial services industry. I’m not saying that with rancor or hyperbole, it’s simply the way it is.
Also, be sure to use the opt-out of arbitration site or do it yourself, or you may be out of luck for any redress when the inevitable class action begins.