Is there any social network website on which I can find people who match my interests (unknown people who are not already my friends), also the content (videos, articles etc) that match my interests?
The first ones that comes to mind are three sites with different purposes.
Xydo is a new social site that is similar to digg, but way better. It aggregates all types of news sites/blogs and allows you to vote-up news stories and lets you 'subscribe' to your interests. Subscribe meaning that those news articles will show up in your news feed on the homepage. I love it. However, for the newer users it's invite only for right now, but you can always sign up for a beta account.
LinkedIn is a social site for business. I use it because there are potential clients or business partners that I can find in case I needed that. They also have a Q&A section, but it's much to be desired.
Quora is the Q&A site for interesting questions answered sometimes by the best and the brightest in that particular field of the question. It's social too as you can share and follow, but give it a whirl as I'm sure you won't be disappointed.
Using all those three sites should give you a balanced platform to find anything you needed in your field.