And also another one is Elgg I believe it is called. I would check those out. Also keep in mind you're attempting to break through in a social network world where there are millions out there trying to do the same as you. You def. have your work cut out for you. In no way am I saying NOT do it, just letting ya know you gotta make sure you create your site strengths with other sites weaknesses.
I find that OOP in PHP is bare-bones, this leads me to believe that PHP is only suitable for the smallest of projects. On the other hand- Java seems to be good for only the largest of projects. A social networking site is a very large project.
I wouldn't think twice about which one to choose (I would use a Java servlet), but I already have a taste of both.
On a large project you'll find yourself battling your own code- it will be very hard to maintain.
I use (, ; : -) as I please- instead of learning the English language specification: I decided to learn Scheme and Java;
You might want to take a look at the book PHP 5 Social Networking (my review). As far as whether PHP is optimal, you can probably get as many opinions as you ask different developers, but it is what FaceBook is built with -- though admittedly they have have used their "HipHop" conversion tool to optimize performance by converting it into C++ executables -- but they have also made HipHop open-source, so you could use it if and when you ever start to get within an order of magnitude or two of their user base and traffic.
"Please give us a simple answer, so that we don't have to think, because if we think, we might find answers that don't fit the way we want the world to be."
~ Terry Pratchett in Nation