Very good thread. It totally depends on context and the psychology of the people you are marketing to. Also, another factor to consider is how much free content/information is already available for that particular mainstream market segment.
Also, the definition of spam varies depending on who you ask. It can be as restrictive as merely putting a link with your communication (I say communication not just email or post or chat message because there's tons of differing ways to communicate online) or narrowly defined as email spam. Fortunately, the places you hit will also have varying degrees of restrictiveness regarding their definition of spam.
I'd say the Terms of Service for the site is a good place to see if something is spam or content.
