Ask HN: Ways to secure a mobile app's API back end?

Hi there,

I am implementing an API backend for a mobile app and I want to know how can I secure it. Users won't have usernames and passwords, they will signup on the mobile app using Facebook/Twitter.

Is passing OAuth tokens with the request to the API considered secure? What happens when an OAuth token gets stolen or spoofed from the network?

Thanks for taking the time to answer.