Are you seeing ads on Facebook? You're in the wrong place. Please see this Knowledge Base article instead.
Are you seeing ads on YouTube? You're still in the wrong place. Please see this Knowledge Base article instead.
What about those other things you don't want to see, such as Facebook's "People you may know" and "Pages you may like," and YouTube annotations and comments? You can eliminate those too, just by subscribing to some additional filter lists.
Keep in mind, however, that every additional filter list you enable in AdBlock slows your browsing a little bit. We recommend that you use only the filter lists you absolutely need.