Exactly, by engaging in lots of other people's comments/contents in both Instagram and Facebook is one way of getting noticed and once they check out your page and see good contents, they may follow you. It's how I got my Twitter account to have 3k followers.Concerning promoting your Instagram profile and Facebook page without advert promotion. Well, there is one thing I am certain about for you to promote your account, but it will require you some spending because people like when there is a reward for a task, so you have to do some giveaways . You also need to reply posts related to what you do on your page, that can also attract people