The simple answer is "yes" - but you have to do it the right way and even then it's not necessarily the best option.
Commercial activity, but only on certain parts of the site, ie, Facebook Pages (aka Fan Pages).
They don't want you doing it through your Personal Profile or through Groups. Links on other people's walls or in personal messages are likely to be disabled as spam, plus they get a poor response anyway.
The most successful FB marketers don't use direct linking anyway. The basic formula they follow is this:
1. set up a Fan Page or Group
2. provide people with good content and links to good content to:
i) encourage people to join (the group) / become a fan (of that page)
ii) build a relationship with them
3. market their members / fans outside FB
The way to market people outside FB is to offer them an ethical bribe (eg Free Report or video) to get them to sign up to your autoresponder list.
Once they've signed up you can market them as often as you want without having to worry about FB.