How do you draw the line between effective promotion and spam?

This is a question I've asked my LinkedIn network, and I also ask here:

How do you draw the line between effective promotion and spam?

When marketing a brand or doing some other type of publicity for someone or something, there are effective ways to promote, and there are not so effective ways. How do identify, especially in the digital age in which we live, where the line between effective promotion (self- or otherwise) is and where you've crossed it?

Comments are open and genuinely appreciated!

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crossposted to http://getthatjob.blogspot.com