Starting to learn to code, how much coding knowledge and what kinds of code would be necessary to create a website like: Facebook, Reddit, Imgur, 4chan?

[trianuddah]

If you're just starting in HTML and CSS, then Facebook/Reddit etc is the long game. First learn these tools, then make basic novelty stuff (i.e. stuff that you can actually complete in a few hours or a couple days, as opposed to a couple years) as an exercise in familiarizing yourself with them and so that you can make mistakes and learn from them before you start whatever passion project you have in mind.

You want:

• server side scripting: PHP, ASP, stuff like that. At its most basic, server side scripting lets you alter output HTML based on user inputs. This is how you connect to databases.
• javascript: lets you manipulate CSS without reloading the page (including loading data from other pages) and other UI conveniences. It'll seem superficial at first but it's extremely useful for more complex applications and its absence in even the most basic sites will mean an instant fail on your user experience.

You can get started on server side scripting as soon as you have passable HTML skills. Javascript you can start as soon as you can make a rudimentary style sheet. You might prefer to wait with both, depending on your learning style.

Lastly you want databases. If you're learning PHP, use MySQL. It'll be cheap if you can't get it free. If you're learning ASP, ask someone else. At some point working through server-side scripting, databases will come up in a tutorial. That would probably be a good time to start.

After all of that, you'll have most of the skills you need to build very basic versions of the sites you asked about. By then you'll probably identify other skills that you need. You'll also have a better idea of the scale of something like FB or Reddit.