How to Filter by Time
If you would only like facebook access from 2pm to 4pm every day (14-16 in 24-hour notation), here is how you could do it with Pluckeye:
pluck add "Sometimes 14-16 Allow facebook.com"
Or, more stringently:
pluck add "Sometimes 0-14&16-24 Deny facebook.com"
See eg.plu for complete rule syntax.