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 + when "14-16" allow facebook.com
Or, more stringently:
pluck + when "0-14&16-24" block facebook.com
Or, for only on weekdays:
pluck + when "MTWRF0-14&16-24" block facebook.com
The schedule format used by scheduled rules is not well documented, but perhaps the following legend will enlighten.
See https://u.pluckeye.net/hrs3 for a point and click interface for creating schedules for Pluckeye.