basically what I want to do is forward people to a download link once they either 'like' my page on Facebook or post a link of the page to their profile (whatever is easier) and something similar for Twitter.
I have seen some bands do this when promoting a free download — to download the new song you must post this to your profile etc.
Anybody know how I could go about this? (This isn't a 'can you do it for me' question, I just need a point in the right direction regarding API's or any examples)
Thanks.
Look at the edge.create event in Facebook javascript SDK. This hits after you've liked something.
You can also create a Facebook app and check the likes of a Facebook user with their permission on the server side using FQL (user_likes).
You can create a Twitter app and read the last few tweets to check if there is a tweet containing your text/link/whatever programatically.
Notice that all of these can be deleted easily after you've redirected them. You cannot avoid that.
This Page has the like button attributes. Look under FAQ and you will see "How do I know when a user clicks a Like button?"