What kind of features does Facebook and Instagram API's offer?

[Jade]

Basically, I'm interested in how deep the Facebook and Intagram API's let you go.

For example, could one simply build something extremely similar site to Facebook off their API?

I have noticed many plugins, extensions, apps and more that allow you to view your news feed on both Instagram and Facebook, along with notifications. Although, I have not yet come across one that actually lets you post to either Instagram or Facebook. Is this because that specific functionality is not offered because Instagram and/or Facebook could be at risk if they did offer this?

I did come across a small extension on Wordpress that allowed users to stream their blog posts directly to Facebook. Is something like this also possible for Instagram? Could this functionality be turned into something that allows you to post a status directly from an extension (as a Chrome browser Extension for example).
Inda Cloud... I asked for information, not links that I already know about.

[Inda Cloud]

http://instagram.com/developer/#
https://developers.facebook.com/