Assuming NSA is primarily targeting jihadis, one of the explanations is that a preferred technique is "throwaway" online accounts, with the login details passed

Assuming NSA is primarily targeting jihadis, one of the explanations is that a preferred technique is "throwaway" online accounts, with the login details passed between users. This prevents the content from going between service providers, and arguably within Google is probably more secure than most Internet email. Thus, one account might be used for one conversation by multiple parties, or even one message. (Plus, a user might use dedicated accounts for communicating with specific other users, as a form of compartmentalization, and of course a cell or network probably has multiple members.)