I have done tons of Facebook ads of various types (top of mind awareness, call to action, etc). I seldom find success in using them to boost a slow day. The reason being is the approval process takes a while, and by the time it actually starts appearing, most people have already had or planned their meal.
Instead I keep constant top of mind awareness campaigns going, with different ads catered to different demographics and markets geared toward gaining page likes and follows more than immediate orders. I usually include a "Make sure to follow us on Facebook for exclusive limited time offers.
Then, when I have a slow day, I put a “Flash” offer out with Limited Time Only. Something along the lines of “This deal will self destruct in X hours”, or another urgent catchy phrase. These hit instantly to those actively on Facebook, which most people go on at some point in the day. Once the time, or order count, has passed I delete it.