Sort of like a credit score matched with landlords who vouch that you rented from them before. The longer that history the harder it is to fake and also the harder it is to just be the same person with multiple accounts. These systems usually have some component of a social graph where people will interact with and vouch for other people and having more and higher quality connections (more verified, connected to someone you know personally is real) gives you a higher legitimacy rating. It's usually up to the organization to set the minimum bar of how verified someone is to be considered unique.