Without Facebook Instagram wouldn't exist since Instagram used Facebook's social graph aka "Log in with Facebook" in order to kickstart and grow its userbase.
Instagram was growing like crazy[0]; it launched in October 2010 and by April/May 2012 it had 50 million users. So they gained 50 million users in one and a half year. There were other photo sharing apps as well at the time on the Iphone so the growth is even more impressing.
Future for them was bright but they needed more engineers and more professional management to cope with the rapid growth since they had only 13 employees at the time of proposed acquisition which eventually happened. Zuckerberg said Instagram was about to hit the wall if it wasn't Facebook because their backend wasn't good enough for millions of users but if they got more engineers and a good CTO and a good COO they could've survived and grow on their own.
