But then so many people on FB are legends in their own minds and curate their profile to scream so.
From the information coherence perspective, the requirement of longer running, more consistent public information with independent public validations (interactions, relationships, location presence, etc.) makes the personas not concurrently replicable, transferable or reusable. (Not taking into account creating fake social media cross referenced identities on the fly which is done for law enforcement and undercover purposes but not for deep cover.)
The required exposure and public undercover networks are also mutual damage multipliers. Once burnt they are operationally finished with cascading affects. Add instant facial recognition, biometrics and a whole network can get nuked before they know. The technology is not future, it is operational now.
In the age of social media, where the complete absence of a detailed social media presence can be a red flag, how do spies conceal their identity?