As you may be already aware, Facebook has revised it permission mode by deprecating the offline access permission.
Under the new model, an access token will get expired only after 60 days it is granted. To renew the access token, based on the document, if you are getting that at the server side, you can simply renew that by making an request to Facebook.
To certain extent, it is unclear. As if based on the document, I can simply make a query on “/me” once every 6 month and that means the token will never be expired.
However, the true picture is that in order to renew that, an active user session is still required. In other words, you have to ask the user to access your application / website pages explicitly. This is something similar to those “please visit this link to keep your account from being deleted” mail that we have received from some free service providers.
Hope this clarifies.