It's been a long since I removed all my Facebook App for Android. However I still can see "last seen" timestamp. How? I have two FB accounts, A and B.
A -> chat turned off, no app sessions in Secury except for the actual desktop session.
B -> incognito mode, chat on, can see A seen "1 min" ago and every time A use facebook.
As far as I know, this used to happen only with mobile apps.