In "create a checkin" from the API Endpoint page, I see:
=====>>>>comment The attached message for this checkin. This is a
great episode if you're a scrimshaw nut.
facebook Determines if the checkin should be posted to facebook. If
this parameter is not specified, behavior defaults to the user's
defaults. true
twitter Determines if the checkin should be posted to twitter. If
this parameter is not specified, behavior defaults to the user's
defaults. false
Is'nt what you are looking for?