For this work you have to use an app for mobile, right now people use Facebook more than mobile phone.
How to Hide Last Seen from Mobile?
Step 1. First of all you have to install an app named Unseen No Last Seen, which is the app, with the help of this app you can hide last seen not only Facebook but also WhatsApp. So by clicking on the link below, install the app on your phone.
Step 2. After installing the app, open it and then enable Unseen Option.
Step 3. Now you have to click on Setting Option.
Step 4. Now you will get to see many options in the page that will open in front of you, if you use Facebook as well as Whatsapp, Telegram, Viber and want to hide the last seen of those apps then all the apps. Check mark means Enable.
That’s it, now you open any message on Facebook, read it, no one will know that you have read the message sent by him because the last seen option has been hidden.