facebook/php-sdk — comment on commit 2b19d803635b36d0ad06faac4e253c83dcbf68aa
File:  line 0
By oyvindkinsey:

They are not - destroySession is only used to invalidate cached data (though can be invoked during or after logout due to the session being invalidated).
We now set the user to null instead of 0 as 0 would get 'stuck', not allowing you to restore or add a new session.