Presence (and notifications) it's an interesting feature for
a service like AFCS, because of the way we implement accounts.
I mean, if you have an SDK account that you use to runs your
applications, would you want a system that gives you "global"
presence (i.e. access to every user in every account) or limited to
only your account ?
If you think global presence is required, how would you
identify users that are not in your account ? Especially if your
applications, or other applications for which you may be interested
in some users implements external authentication in such a way that
user ids have nothing in commons ?
If you think you could live with local presence, then maybe
implementing the lobby as an application running in your account
may not be generating heavy traffic/processing because you would be
dealing with a much limited number of users.