Facebook, YouTube and other social networks provide social buttons and similar features which we use on our website - such as the "Like" buttons or embedded videos.
To do so we embed code that they provide and we do not control ourselves. To function their buttons generally know if you're logged in; for example Facebook use this to say "x of your friends like this". We do not have any access to that information, nor can we control how those networks use it.
Social networks therefore could know that you're viewing this website, if you use their services (that isn't to say they do, but their policies may change). As our website is remarkably inoffensive we imagine this is not a concern for most users.