Bitcoins can certainly fall. How much it falls depends on your calculations but a good estimate is to use the % change in price over total price of historical price change data compared to figures like volume of trade and such. Right now it's tough for anybody to figure out when the recent boom is going to stop but it seems a great number of people are starry-eyed. A crash? I couldn't even begin to tell you if that could happen, especially with so many investing not only their money but their time through the huge number of services like merchant websites, escrow services, ease of payment services for bitcoins, gambling sites, all sorts of sites. Once people invest not only their money but their time and interests, a niche market like this is usually around for the long run.