Technologically, it is very easy to start a competitor to Facebook/Twitter (the scaling is the hard part, not the starting). So, there will always be competitio

Technologically, it is very easy to start a competitor to Facebook/Twitter (the scaling is the hard part, not the starting). So, there will always be competition until the biggest players can somehow raise the cost of entry by a few orders of magnitude. Socially, I don't think it is that hard for people to migrate from Facebook/Twitter or something else. I think a lot of people think about competition in this area happening like a huge number of people stop using Facebook/Twitter and start using a competitor almost simultaneously. That is kind of what happened in the past, but only because the old social networks had serious problems. Now, significant shifts will still happen but they will be a lot more gradual. A lot of people are on multiple social networks already (e.g. Facebook, Twitter, GitHub, Mixi, Linked In, Flickr). Over time, each user's the proportion of page views on each site changes gradually but often significantly. Facebook/Twitter might remain the site (almost) everybody has in common, but I doubt it will continue to enjoy the huge share of users' attention it has now.