It depends on how many clicks your site gets. If you are seeing a reasonable amount of clicks then CPC will perform well for you. If you have little to no clicks I would stick with a straight CPM network. Some networks can combine CPM, CPC and CPA.
Google adwords is good for user to search a keywords so depend on the product that you want to sell facebook depend on the people that he like or dislike depend on their community. as one of social media, facebook marketing depend on "people tell other people, but not depend on keyword. PPc depend on you , you must select what product you sales.
It depends on many things. It's a great method of advertising if you are selling a product or a service, but if you have free content on your site, there are much better ways to promote your website!