There is no doubt Facebook dominates the social networking world. One can't help wondering if they are taking a bite out of smaller competing websites traffic. Just how big that bite happens to be remains a good question that would be difficult to answer.
I can only assume that with all the Facebook addicts out there they are probably not spending as much time signing up or joining smaller sites as much as they used to. If a website has a sign in with Facebook or Twitter feature that allows them to do so in a couple clicks you might see that happen. If a website requires someone to fill out a bunch of details they might not bother. People are lazy these days. If they can't accomplish something in a few seconds they have a tendency to leave and go elsewhere in a hurry.
A website that resembles Facebook or has the same look with the same or similar features I think will have a tough time getting new members. Whether it is a Boonex Dolphin website or any other type. It would have to be extremely popular and talked about. But, if it is so similar I can't imagine it being the talk of the internet world. You never know though anything is possible.
I think a website that specializes or caters to a smaller group of people might have a better chance of success right now. If you are trying to create a general social networking website you are in a manner of speaking competing with Facebook. Even more so if you are creating one that has a similar look and feel to it. If you were to create a website about a particular game, a game system, technology, SEO, maybe Sci-Fi, books, or some kind of specific hobby you might attract a few more people. Of course there is no guarantee and you really never know. Just don't come up with something that nobody is interested in or has never heard of.
Those are my thoughts for the time being anyway.
The internet is constantly changing. One moment a website is hot and popular and the next something else comes along and people flock to see what it's all about. Sometimes they stick with it and it continues to be popular. Other times it doesn't last and it dies off.