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:
What other strategies could be useful?
Target a niche audience that will see great immediate value out of
it will get the ball rolling and require little effort on your
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.
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.