The service is Facebook Advertising, but - this in itself is made up of several processes, including:
- Setting up campaign plan
- Setting up and checking tracking
- Building messaging and creatives
- Launching campaigns
- Split testing
- Troubleshooting campaigns
- Scaling campaigns
Each one of these is it's own process and some have many processes within them. You should only be selling services to the solutions you have a process for.
For example, if you only have a process around creative and copywriting for the ads, then this is the service you should first sell until you have written out step by steps for the remainder that make up the broader "Facebook Advertising" service.
If you're offer requires a lot of creative skills that may be hard to list out, try to map out the steps you take to help you develop whatever creative outcome is needed so that all you now need to do is recruit people who share a similar eye to you.