Yo guys!
Last week I noticed a few ad sets with significantly higher number of Post Saves that others. In most cases this were ad sets where FB algo pushed more on IG than on FB. It's much easier to save an ad on IG than FB because user only need to click on the icon.
Anyway, here is how you can retarget users that saved your Fan page or Post/Ad.
1. First open Account Overview and select predefined setup Engagement.

2. Inside Audience Manager and under Facebook page there is option to build an audience of People who saved your Page or any post.

3. Run some testimonial ads or any other retargeting approach it's working the best for you

Stats from the Scaleship project for the past 5 days. Not too shabby. 
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I'm sharing this here because I believe I'm not the only one that didn’t know about this audience.
Hope this helps.
Cheers,
wakeboarder
EDIT: You need to build different audiences for Facebook and Instagram. See below.

Damn bro, beautifully simple and well spotted!
Those ROAS are so juicy - incredibly low add to cart values lol! All those so close to buying finally got in the retargeting net.
Another great tip Timotej! Thanks for sharing it with us 
That's a good idea. Thank you for sharing.
Another good angle to try @wakeboarder is Retargeting on email opens and email clicks too :P
Afuckingmen. Like maynzie said juicy ROAS