Growing follower count: Quite aggressive plugs for other accounts, but I would imagine the last section might actually be a great way to grow an audience with Instagram.

Twitter followers: Here’s a low maintenance way to increase Twitter followers; at Sets you sig up using Twitter, so the only reasonable thing is to ask the user if you want to follow their Twitter account right then and there.

Follower marketing: @hackernodeapp is doing the “follow people on twitter that might be interested in me” thing, a viable strategy if various elements feel right: A) Profile description, B) Followers/Following ratio, C) Social Proof

P.S. I am a HUGE Hackernews fan

Extend the users’ social graph: Angellist (@angellist) helps you finding relevant people by automatically signing you up to the people close to who/what you just followed (don’t stretch this though, it works here but may not in many other cases). 

Increase following: Tapbots (@tapbots) collect followers in the setup stage for their iPhone twitter client Tweetbot. 

Friending: on Instagram (@instagram) you can find existing friends, followers etc. on social networks like twitter and facebook. However when you’ve found and added them, Instagram still periodically checks whether someone you know on these networks signs up later and lets you know in the news tab.