seems like he is searching for some complex functions. If you compare it to twitter, facebook etc. you will need something more than just "HTML and CSS" These arent language for complex webapplications and websites.

Based in your needs you need an content managing system or a completly new programmed website. If you dont give some further informations its hard to give you a answer

If you are searching for php-programmers (a programming language to do complex websites) you can look on portals like www.scriptlance.com.

Big companys like facebook and twitter normaly have their own programmer teams which works just for them. But for that you will need a big budget and for a big budget you need a really nice idea..

Greetings from Mexico