I think this question is a bit wider in base but yes answers are multiple for different roles. Like in my Case as a Blogger I market myself on Forums, My Website page on Social Networking Sites and by involving myself in discussion with fellow bloggers on various online platforms. As a Professional I keep in touch with my fellows and ex co-workers by being interactive in Linkedin, Facebook, Whatsapp and BBM.
Great question. Nowadays I started a new product (http://toptab.net) which is B2B. So I do some of the usual stuff as others will probably say as well: Forums, Email, using connections because its B2B after all, not yet SEO because I its a new type of unique product. But mostly I try to think outside the box. So I created a G+ page were I recommend about Chrome extension (its kinda related to my product) so I collect there people, give them interesting information and later-on I will also push my product. bottom line: think about other ways then PPC,email,forums, etc. Go for Chrome extension, Fan pages not related directly to your product or any other new approach Hope my comment helps