By Josh Bernoff, from my Marketing News column.

Social technologies take a while to build, but last a long time. Think about the effort it takes to get people reading your blog, following your Twitter feed, viewing your YouTube videos, joining your community, or friending your Facebook page. They all start with zero viewers, but the more they grow, the more powerful they become.

Ad campaigns move at a faster pace. More importantly, they have a beginning and an end. You rent a chance to get some attention for a few months, then you see whether you moved the needle.

Since advertising people often get responsibility for social elements of marketing, this creates a fundamental disconnect. Marketers who tap into these two forms of communication can get whipsawed – the social builds too slowly, and the campaign ends too quickly, to make it easy to synchronize them. Even when they do succeed, there’s huge waste. If you’ve assembled 100,000 customers into a community behind your brand, what happens when you’re done with them? Send them a thank you email and say good bye? That’s a tragic waste.

The answer, as my colleague, Sean Corcoran, discovered in the research behind his report “Using Social Applications In Ad Campaigns,” means thinking of social fans as an asset that you can build with a campaign and then tap over and over again. To do this, you must also make sure you connect with and feed them between campaigns, to keep them interested.

For example, Starbucks has 1.2 million fans on Facebook. So when it launched its first instant coffee, VIA Ready brew, those brand loyalists got to tag whether they liked it, and to share it with their friends. And H&R Block took its 2008 tax-time promotions on MySpace, Facebook, and Twitter and turned them into 1,700 Facebook fans and 2,300 Twitter followers that they regularly check in with as tax season approaches.

Marty Collins, a social marketer at Microsoft, has figured this out. She supported Microsoft’s “I’m a PC” campaign late last year with short-term social tools, so people could contribute their own content to the campaign. But now, Marty spends 75% of her time with continuous online discussions to make sure the company has an engaged audience to tap into for the future.

Here’s how to master the rhythm. First of all, you need a launch plan. Sometimes, this means identifying key fans and tapping into them early, so they can help you build a community, spread buzz, or make the contributions that will attract more people to your offering. It may also mean starting the online portions of your campaign early, so it has time to grow. If you must start your online social campaign concurrent with your other ads, be aware that most of the impact will come weeks or months into the campaign, when the momentum and word-of-mouth has time to build.

Second, during the campaign, pay close attention to customer participation. Social applications need fuel. You may need to plan events over a period of weeks or months to keep people coming back. And don’t be afraid to make mid-course corrections in your social campaign, based on what’s working and what’s not. Most importantly, your company has to participate. Since you’re tapping into brand fans, the feedback from you is the fuel that will keep things going. You can’t spin up an online community and expect it to continue on its own.

Finally, you’ll need an exit strategy. What will you do when the campaign winds down? If you’ve named your Twitter account after your ad campaign, you could have a problem – maybe it’s better to name it after something more permanent, like your brand. And do you have the budget to keep the community going in a maintenance mode? You’d better budget for that, because this community could well generate the ideas for your next campaign.

In the end, companies that have built up fan bases this way will have an advantage. For one thing, as this trend picks up, the socially active consumers will be getting serenaded by every brand in America. In this environment, Nike, Coca-Cola, and Scion are all direct competitors – better to sign them up to your dance card before another brand comes along to romance them.

Of course the real challenge here is for agencies, who tend to get paid on their short-term efforts. Partly because of this short term focus, clients are reaching out to digital agencies like imc² and PR firms like Edelman and Fleishman Hillard to build social applications, often keeping them out of the hands of traditional ad firms. Agencies who wish to play here will have to adapt to these long-term platforms, which means a new set of skills.

They’ll also have to find ways to get paid for efforts that pay off in the long-term. To that end, marketers ought to consider paying by the number and enthusiasm of fans recruited, rather than by the impression. That’s the only way the agency, the campaign, and the social media will ever end up in sync.