Yep.
There would ofc be more than just emails on an office-environment server. Many of the "clients" would have been using desktop fb, twatter etc.
Before Zuckerberg made his committee appearance, there were rumours that ppls private msjs from MZ were "disappearing" from their inbox. Must look back for sauce…
Old FB rules for msjing here:
https://www.facebook.com/notes/facebook-engineering/inside-facebook-messages-application-server/10150162742108920/
Metadata from the carrier though…thats who REALLY has it all…