You have to start somewhere. .NET is out because IMO Java is more open standards based than .NET. PHP - because it's fairly quick to learn and write in addition to being fast.
I haven't looked at all options yet, but I'd like to get more opinions on Java vs PHP first. You are welcome to suggets more.
PS: Friendster rewrote their site in PHP (from Java)
Use what you know best. I don't like the feel of writing Java personally and never had to take any Java programming classes to force me into it. PHP on the other hand is very easy and natural to write, but the code can end up damn ugly. I also haven't looked into it too much, but I think Java beats out PHP for performance speed. Both have pros and cons.
If it were me I'd suggest Python. I need to look into web.py some more, but I currently use Django for most web things. It's easy to write the code and doesn't end up looking like ass so it's easier to maintain.
I haven't looked at all options yet, but I'd like to get more opinions on Java vs PHP first. You are welcome to suggets more.
PS: Friendster rewrote their site in PHP (from Java)