There, now. You see that? Facebook says it didn’t mean to keep tracking you after you logged out. The bad mistake is gone from the code, and Facebook is as safe and happy a web site as it ever was. What were you all worried about? Let me ruffle your hair and send you outside to play. Does iddums want a cookie?
Nik Cubrilovic Blog – Facebook Fixes and Explains Logout Issue.
Actually I have read somewhere that even after logging off from Facebook it keeps the session cookies unchanged and hence it gives rise to a vulnerability which is being easily exploited by hackers.
So, I believe Facebook should do something to avoid this thing if they want to survive otherwise Google plus will be the option.