John Grant has a great post on this topic. Anybody who is in the business of building brands or in marketing communications must read it. They must read, adopt, practice and evangelize it.
Here's an excerpt from the post:
So what is ENGAGEMENT?
It is obviously related to a new set of tactics and media; to online communities, experience events, word of mouth… But it isn’t just about adding these to the media plan. It’s also a different kind of strategy.
Fundamentally: Engagement is about building RELATIONSHIPS over TIME (not one off sales)
For example
TESCO COMPUTERS FOR SCHOOLS
THE HARLEY OWNERS GROUP
EBAY POWER SELLERS
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