Interesting question. My take would be that you need to toe the line between building something quick and dirty, and building something that can scale. If you can't get the features right then scaling is a non issue. If you do get them right, the nature of this type of business is that it will grow quickly - whether you like it or not. So you have to be able to add scale quickly or it can fall apart due to unavailability (people often attribute friendster with this outcome). These might be the hardest businesses to start, because in addition to cracking the chicken/egg problem, you have to steer a fine line between development speed and scalability.