It depends on what you want the new users to do. Ads mostly bring visitors who don't know about your company. If you are in a business where branding or trust is important, it's more difficult. If you are in a commodity business where prices matter, I think it's easier.
That's normal they have a different behavior. Comparing Google Ads and Facebook Ads is hard as there is a user behavior difference between them.
- Visitors from Google Ads often have a purchase intent - Visitors from Facebook Ads are currently entertaining thanks to Facebook photos & videos. (you need to create a need, a urgency to sign up / purchase)
-> my startup has experienced that. Google ads bring qualified users to Moneytis.com (comparison of money transfer options), whereas Facebook ads bring curious people to sign up on neomy.io (fun alerts about currency exchange rates)