Sounds right. Get the ball rolling with friends, Facebook ads, give-aways, following your audience everywhere and so on. Then just stick to, be consistent, and let the snowball effect take over – provided you actually create good content people want follow and share.
Some of the mentioned methods are a bit shady though. The follow-to-unfollow practise works, but is of course a type of spam. And scraping email addresses for commercial purposes? Well, I would get sued for that here in Europe.
Depends on the project, but almost always Facebook and Twitter, sometimes also Instagram and Google+. It’s those places where people go every day, so you have to be there as well to get their attention. But that’s about it. In the end I want them to become daily visitors of my site(s), email subscribers, YouTube subscribers and so on. I much rather have full control of everything on my own sites and don’t want to rely too much on social media accounts.