Surprised that the article doesn't mention Website Custom Audiences (WCA in short), which for many startups might be much easier to collect compared to email addresses.

With WCAs you can build audiences based on people who visited your site (or say, a specific URL like a blog post or a thank-you page). You can then use these both for retargeting (e.g. someone visited your site but didn't sign up yet), or build lookalike audiences out of website visitors and/or signups. Imagine that someone read an amazing blog post describing some key features or use cases of your product? Why not create a WCA and then craft a creative addressing that specific audience segment.

Another benefit of WCAs and lookalikes based on those is, that they're updated automatically, whereas for email-based custom audiences you'll either have to manually upload new signups to FB, or set up some custom automation to do that for you.