Facebook Dating App is a feature within Facebook’s major App instead of a separate app release.
Facebook users create a separate dating profile. After showing interest in another profile, the service lets users contact each other.
Facebook Dating and its attendant secret crush feature would not be available in the US until the end of the year It has been launched with a test version in Colombia in September and is now open to Facebook users in 19 countries including Canada, Mexico, Brazil, and Thailand.

What is Facebook Secret crush?

When Facebook Dating is available in your area, you will be able to register by responding to the notification in your news feed.
You can also opt-in by clicking the heart icon on your profile:
This will bring up your “dating home”
Another method of how to sign up for Facebook dating?
Once you open your profile. Tap on the Heart icon to enter what Facebook called “Dating Home.” From there, set up a dating profile, which is not going to be seen by your Facebook friend? Surfing events around you, groups that interest you. And unlock the physical world event you will like to attend.