Reaching critical mass is becoming harder. What are some innovative ways to expand your user base?

There is certainly not enough attention on this subject, yet it should be a real concern to entrepreneurs.

Basically, you need to be aware of other means needed to kick-start traffic, especially if you have a site who's user experience depends on the other users. The first problem is the chicken or the egg problem, and further on you reach the problem of getting the site into full blown disease mode.

Guerilla marketing, giving incentives to users (paypal), and ethical to semi-ethical spamming (myspace,facebook) are not out of the question. Theres a big difference between using these tactics to get initial traffic, to actually basing an entire business model off of their continued use.

If you have a niche site, or one that isn't as catchy, you're probably going to be doing all and even some new methods of kick starting that aren't even known to you yet.

SUMMARY: Getting a techcrunch article or some alpha blogger love doesn't always fix chicken or egg problem problem and you might very well be doing some of the methods stated above. On the bright side, this can create a minor "barrier of entry" and can keep other startup founders from emulating your idea over night.