When developing an online system the choice of communication mechanism isn't usually something that is given too much thought. It is common for users of online systems to receive "virtual pokes" by email when others interact in a way that directly affects them.
The nature of online applications, with users logging in and out at different times, requires a mechanism for syncing the actions of disparate users. However, it is always worth considering if the method chosen is actually the best way forward. One factor that should be considered is the volume of communication that an individual user receives - quite often users need help in organising large amounts of data and leaving them with an overflowing inbox is not helpful to the individual or the system as a whole.
One of our recent projects involved an online back office system handing thousands of user requests a day. The system administrators (of which there are less than a handful) had the job of manually validating some of these requests before they were proceedable. Our initial implementation of this system informed these administrators of each such request by email. Please note, this was entirely at the suggestion of the administrators themselves!
After a couple of months of having their email inboxes overrun by these emails we realised that this was the wrong communication mechanism for this process. Emails were being ignored, adminstrators were struggling to cope with the constant pressure of emails arriving every few minutes and the whole process was moving far too slowly.
By moving the process outside of the online application itself and into the email boxes of the administrators, the system had become too dependent on each individual's organistational skills and their workflow had become disjointed. Every time they received a request they would have to follow a link to sign in and then approve the request - this was an unnecessary overhead which, when multiplied by the sheer volume, resulted in a lot of wasted time and effort.
To resolve the situation, we provided a virtual inbox listing these requests as part of the online system from which the adminstrators could browse and approve requests easily and quickly. They could log in a couple of times a day and clear out their list with greater speed and far less stress. As a result the requests were processed much more efficiently for end users and everyone was happier with the system.
The lesson to learn from this is that whilst email certainly has a role to play in engaging users with online systems, it is not always the best choice. Careful consideration should be given to the communication mechanism to ensure that any online workflow is not hampered by it.