I would like to post to a user’s Instagram or Facebook page (after they approve). How do i do this?
Anytime you’re interested in integrating with another service, the first thing you should do is look into that service’s documentation as the best source of truth to what you can and can’t do. You may know that Instagram only permits its users to add photos via their official mobile applications, which unfortunately translates to developers as well. On the other hand, Facebook has a plethora of information pertaining to the creating and accessing of objects such as photos. You’re likely going to be interested in Facebook’s User Photos endpoint of their Graph API. If things get too complicated to work out yourself, you can always explore services such as Zapier and Blockspring. The most important step you can take is to just get exploring.
You are going to need a “publish” enabled token to be able to access that endpoint.
Which could prove interesting, unless the FB login / OAUTH functions now allows that.