Strategies for gaining mass adoption for a web startup

Say now I develop a web site that requires a large number of users to work. eg. StackOverflow which requires collaboration between many users.

Whart sort of strategies could I employ to gain mass adoption:

1. Implicitly, the product must be great.
2. Use any existing marketing channels to get the word out. eg. If you have a popular blog use it as a marketing channel for your new product.
3. Make an initial investment upfront to intentionally employ users to use the site. This could be a catalyst to attract more users. Who wants to join a party when there is hardly anyone there?

What other strategies could be useful?

1. Yep, but there are many great products that on one hears about, and MANY crap products that everyone hears about, so 1 shouldn't even be a focus. Just make sure it's something people want to use, not necessarily a great product that people want to use.
2. A popular blog is not going to make or break the product, just boost it's adoption depending on audience numbers... Target some popular blogs that accept reviews and write a review for them that they're likely to post. Though I don't think anyone has ever given fame to a single blog for building their business, unless it's a news aggregation website.
3. People will help you with little incentive. Zurker (a social network with very little usefulness at the moment) has an interesting plan, which I didn't really look into much but for sharing the link and getting more members, you get points which convert into ownership points of the business. That's taking it to the extreme. You should find that people will start using your site for much less incentive. What is your startup?

More ideas...

Target a niche audience that will see great immediate value out of
it will get the ball rolling and require little effort on your
behalf.

A celebrity evangelist... eg: Ashton Kutcher, Paul Graham, (a popular blog owner again)

Traditional advertising by hanging posters around town. A competition on a popular blog if you're selling a product.

Pretty much what @James Hilton said, but a little of my own $0.02 added in.

1. 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.
2. 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.
3. 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.