You are trying to get people to switch from a service that is already very effective and proven to one that is risky. You will need proof that it works before anybody will pay you money for it, so start by giving it away in exchange for value that you need.

Put together a concise email offering a set amount of free advertising and full handholding in exchange for weekly feedback on the product and the rights to publish a case study after the campaign. Work hand in hand, get them awesome results, then create a report of what you helped them achieve and start to publish these on your site and include them in your future outreach.

Once you have proven yourself with a couple of clients, you can take those results and start pitching people on some form of test that is not free. Offer to help them manage the entire campaign and guarantee some results, but make them pay for it after you have your evidence that it works.

Getting your first few customers involves being very scrappy. Don't expect to throw some ads up and get people to signup for a paid service that's intended to replace something that works very well already. You will need to do things you normally wouldn't until you're no longer an unproven solution.