How do I hide my Facebook last online/seen timestamp?

Facebook active status:

For hiding the active status on Facebook:

• Go to the settings tab in your Facebook (the three lines at the corner of the app)
• Open settings and in the privacy and settings option.
• Go to privacy tab and open the Active Status
• Turn off the slider which says 'Show when you're active'. 
• The notification will pop-up asking you 'If you turn off your active status you may not be able to see others' active status as well'. Tap on Yes.

Now your active status is hidden. Although it hides the active status, you will still appear active or recently active unless you turn off the setting from every place you are using Facebook as well as Messenger. When you turn on this setting, you will not be able to see your friends and contacts active or recently active on Facebook and Messenger