Pretty much what @James Hilton said, but a little of my own $0.02 added in.
- That is the most important thing. Unless your product is good, people will flock to some place else. Even if you get traction for being in a niche early, or being the best of what's available, something new will come up and supplant you. So that should be your number 1 focus.
- Yep, why not? Even if your blog isn't popular you should do that. And use social media to it's fullest potential. Get accounts for your app on sites like Google+, Twitter, Hacker News, Reddit..., and interact with the people there. Get a blog for your product and share news from there. Generate some interest and intrigue about the app. Get a coming soon teaser page on your domain before the launch, and describe the problem you'll solve and what is so great about the product, but without giving too much away. Have a beta-signup, or get people to give their emails for further news. This way you'll have generated a lot of interest before you even launch. Also, if you have the money, buy ads - Google Ads, Facebook, Stack Exchange - whichever platform you think would be the best place to target your target audience. For example if your target audience is programmers, then Stack Overflow would be a better site to advertise on than facebook.
- Well if you get all or even some of the above mentioned things right, then you most likely wouldn't have to take such steps. But if you have to, then why the hell not. Steve Huffman, co-founder of Reddit, mentioned that he and his fellow co-founders made hundreds of fake accounts and posted stuff on Reddit early in the sites life, to get some traction. So you wouldn't need to pay people to use your product, just do it yourself. Though I'd only use this as a last resort, if things aren't as well as I'd like.
Another way to generate interest would be to use Gamification - early users get special badges or whatever. Of course you'd have to incorporate gamification into your entire product, or into your website for it to work effectively, because going in half assed with it would probably not work. Or, depending on your finances, you could have limited issue stuff - ebooks, subscriptions to popular webservices, tshirts and the likes sent to early adopters. That would certainly bring in people, even if just for the free stuff. Team Treehouse, which is a paid app, recently had a 12 Days of Holiday Cheer Giveaway where anyone - members and non-members both could participate, and to improve your chances you had to tweet about it, like their facebook page, follow them on twitter and the likes.
To summarize - There are tons of ways you could get traction, but most important is to focus on #1, and then on #2.
