How do I keep my Facebook Token valid for Video and Post tracking?

Once you authorize the Nudge Facebook App in the Nudge Dashboard you will need to re-authorize the app again every 90 days. 
How does this work?
When you authorize the Nudge Facebook App with your account, Nudge will generate and hold onto a Long-lived token. This token will stay valid for up to 90 days. Once 90 days is up, you will receive a notification within the Nudge Dashboard prompting you to reconnect with Facebook and re-authorize the app. This is the same process as when you first connect with Facebook with Nudge. 
You can learn how to do this here.