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I am developing my site using server side sessions using
redis as backend for saving the session.

Now the issue which is bothering me is of user leaving the
website without logging out. I mean user simply closes the
browser which causes the cookie to be deleted.

Now session of that user still exists on the server and will
not be used again as new login requires creating a new
session due to security reasons. To avoid the case where
hacker steals the old cookie and use it after user login
again with same old session id.

In essence user leaves the website without explicitly
logging out and his session will be deleted after certain
time limit of inaccessibility. I am thinking time limit of
30-60 minutes. Also with every new request from user his
cookie will also be updated to keep track of when the user
last time accessed the site.

But nowadays, people let site remain open for long time
without accessing it. For example users open facebook and
gmail in new tabs and forget about them for 2-3 hours and
still they are not asked to login again.

Is letting a 2-3 hours old cooke access the session secure?
My concern is someone steals user cookie and use it 2-3
hours later.

Thinking on this topic has also forced me to question how
facebook manages security if user can use a session where
they are not accessing it for long periods of time and still
they remain logged in. Or is it not secure for me to keep
logged in when am not accessing the site session for longer
period of time?

It can be the case also there is some pinging mechanism
using which sites keep track of user having their site open
in a browser and when browser closes they are notified and
can work accordingly.

My website is a social network and needs all those security
and usage features which a social network may need.

I am new to web security and web development in general and
may be the case where my above questions may seem a little
basic. If you feel that is the case kindly point to some
good reference where I can read and find answers to my question.