Ideas for promotion ?

Hey guys so I was wondering what are the current promos that you use for increasing user engagement ?
I use the good old limited time discounts, seasonal discounts, limited availability. I heard about the free-pay only shipping promo but I also heard facebook doesn't like those ads(is that true) ? Also do you have any other ideas we can share discuss ?

The F+S thing is bad because your customers will start reporting your ads. Facebook won't tolerate that obviously.

The first methods you've outlined are still working if done right, just avoid being spammy.