Here's an insight from the always-worthwhile Seth Godin's Blog.
Consider that conference attendees lavish attention on the marketers & products they've come to see, whereas when we see advertising for those products in other contexts we probably just ignore it - or actively loathe it.
Which of those environments is better for reaching your customers? One where they've made a great effort and perhaps spent some money to come see you, or one where they're actually trying to do something else and you're in their way?
Here's a quote from Seth:
...advertisers treat prospects online as targets, as victims, as people to subject to interruption. Conferences treat attendees as royalty, as paying customers who invested time and money to be there.
And that's the difference. As long as your site is about something else and the ads are a distraction, you'll see CPM rates drop. As soon as you (or the advertisers) figure out that creating online communities aligned with the advertising, where attendance is a choice by the consumer, then you're creating genuine value.
So in building online communications - should we plan on interrupting the audience with ever-more-stupid ads, or should we create an environment they'll be eager to participate in?
Seems to me the proper use of social media for business is to create an attractive gathering place for your customers - one that's aligned with their operational needs, interests, etc - and become part of the conversation. You will then be a host rather than a salesperson.
Or you could just keep pummeling your visitors with banners. How's that working so far?