From the ping.fm website.
"Note: API keys are meant for application developers. If you are using a Ping.fm app that someone has created, it shouldn't require you submit an API key. Since we've recently changed this process, some of the apps might not have been updated to meet new requirements.
If you are using an app that requires an API key, contact the developer, or visit the app's website for an up-to-date version."
Any word on an update?