You are misunderstanding both of my comments. Regarding network effects the point is that they do make the service more useful (read better) for the users, and

You are misunderstanding both of my comments. Regarding network effects the point is that they do make the service more useful (read better) for the users, and in a significant way, many times more significant than a new UI. So, Craiglist's still better now, if only because they were better before. Unfair? Maybe so, but complaining won't solve the problem or make it go away. It's just that we've been spoiled about never having big barriers to entry anymore. Regarding falling companies was just to counter the parent saying that that kind of companies are unassailable, when they clearly aren't. Nothing more, nothing less... profits or their staying alive being due to network effects are irrelevant to my point. The ultimate point is that just bitterly complaining about how unfair Craiglist is because it's using its competitive advantages to its advantage will not solve the barrier to entry problem, it doesn't even make it clear why is it a problem, nor posits arguments about what to do (if anything) with it.