If you release an API and expect everyone to jump all over it from Day 1, you need to dream on. Building and nurturing the developers using your API takes time.
Here are a few things you can do to get people to use your API.
1) Launch the API with a few companies already using it
Before the API is complete you should start talking to the companies that would benefit from your API the most. Get them interested and convince them to integrate or use your API for the launch. This will do a few things, a) legitimize the API, as people are using it b) create a ripple effect of companies integrating c) press and increased awareness of both your API and the companies that have integrated.
2) Get a good sense of features people want from the API
You may not get your API correct on the first launch. It may be missing a few pieces for some bigger companies to start using. Go out and talk to prospective partners and ask them what they need. Don’t build one-offs, only build if there are multiple big opportunities looking for a specific feature from your API. Build for the masses.
Hackathons are a great way to showcase your API and get feedback from developers in the field. If you are new to the hackathon circuit, I would recommend getting the lowest level sponsorship, demo’ing your API, preparing a workshop, offering a unique/cool prize for “best use of your API,” and hanging out at the hackathon talking to developers. This will give you priceless feedback on your API.