1. A method comprising: (a) detecting that within a pre-defined period of time, a single user is attempting to access, via a single computing device, two or more different accounts of a computerized service; wherein the two or more different accounts comprise at least: a first account that belongs to a first account-owner, and a second account that belongs to a second account-owner; (b) generating a linkage score value that indicates a strength of a real-life linkage between the first account owner and the second account-owner; (c) based on the generated linkage score value that indicates the strength of real-life linkage between the first account owner and the second account-owner, determining whether said single user is either (I) a legitimate human user that is genuinely associated with each one of said two or more accounts, or (II) a cyber-attacker that is illegitimately attempting to access multiple accounts.