Send them a message telling them how you want to connect with them and how you liked their profile. If none of the above methods works, your last bet is to follow their account using your real Instagram profile.
It is worth trying, though! Who knows maybe they don’t have a private Facebook or Snapchat profile. Besides, there is a good chance the person must have restricted their Facebook and other profiles too.
Still, you will not know what the person is posting on Instagram since they may or may not upload the same thing on Facebook.
