1. A method comprising: determining that a particular subscription account of a computerized service, is accessed by two different human users who utilize a same set of login credentials, by performing: (a) monitoring input-unit interactions of pairs of usage sessions that originated from two different users; (b) extracting from the input-unit interactions that were monitored in step (a), a cross-account usage-session pairing pattern; (c) monitoring input-unit interactions of pairs of usage sessions that originated from a same human user; (d) extracting from the input-unit interactions that were monitored in step (c), an intra-account usage-session pairing pattern; (e) determining whether a pair of usage sessions, that originated from a particular subscription account, is either: (i) relatively more similar to the cross-account usage-session pairing pattern, or (ii) relatively more similar to the intra-account usage-session pairing pattern; wherein the monitoring of step (a) comprises: monitoring input-unit interactions of pairs of usage sessions that originated from two different human users and which comprise user reactions to user interface elements that are presented to users; wherein the monitoring of step (c) comprises: monitoring input-unit interactions of pairs of usage sessions that originated from said same human user and which comprise user reactions to user interface elements that are presented to users.