What scripting languages do you have to use to make a social network?

[Jeroonk]

You will have to use HTML, CSS and Javascript, since those are used to build web pages. HTML for the document structure, CSS for the layout and style and Javascript for interactive controls.
These technologies all work inside a users browser.

PHP is a scripting/programming language which runs on the web server. You can use it to make dynamic pages, a single file which loads a different profile page depending on who is logged in for example. PHP is one of the most commonly used languages for this task, but it is possible to do it with other languages or technologies. Think about ASP(.NET), Python, Ruby etc. Each has its pros and cons.

Another thing you will need, is a place to store your data. PHP is only good for working with data and building webpages based on that data, but you still need a data storage: a database. Again, there are many different engines available, but most of them are SQL-based. MySQL is probably the way to go, since it is free and is very well integrated with PHP.

For a starting website, I would recommend MySQL + PHP. Something like a LAMP/WAMP/XAMPP package is probably best to start with. Very easy to set up and get going. These also include an Apache HTTP web server.

There are many different setups possible. Premature optimization is the root of all evil, so do not worry about the particular setup you use right now. If it works for you, it's fine.
But if you are interested, here is how the social networks you mentioned operate:

Facebook uses PHP to program their website and MySQL to run their database. However, since PHP isn't really known for its speed and because Facebook receives an enormous amount of traffic, that becomes an issue. Facebook solves this problem by compiling their PHP website down into one single C++ executable.

Twitter is programmed using Ruby On Rails with parts of it re-written in Scala, to cope with the immense scale at which twitter operates.

YouTube started out as a Python-based website. But I can imagine that they have replaced parts of their site with technologies (probably in-house Google technologies like BigTable for its database) more suitable for the high traffic and scale of the website.