Working with Marketing expert is the best way to go as they know what will get you best ROI.  

If you have clients list already than do re-activation multi touch campaign to reach them.  Doing only one thing is not as effective anymore as people's preferences are not consistent.  

If you are trying to reach new people than FB Ads & Display Ads gets best ROI's for our CPA clients.  

Also adding a BOT on your FB page will also help a lot.  Bot is like your very own customer service rep who collects the leads as well.  It can provide general answers to people's queries or can be build with more features as you like.