Ping.fm Question

Does anyone know how Ping.fm makes calls to all the social networking sites? does each one of those sites have their own API? I can't imagine the smaller social networking sites like Identi.ca having their own open API so there must be a way to find out how to send post or get variables to the SN sites. How it's clear what I'm trying to say, basically i want to make a call to another site from my site I will build, similar to the way you can log into Ping and save account info for other sites to write to them.