If you’re sitting on the sidelines waiting for the market to pick winners in the social network race, you can stand up now. Hitwise data for 2007 shows that MySpace and Facebook together accounted for 88% of all visits to social network sites. The next closest competitor, Bebo , got a little more than 1% of the traffic.

There simply is no more competition in the general-purpose social network market. Other social media winners include LinkedIn (which wasn’t included in the Hitwise data), YouTube and Flickr. If you’re a big brand pursuing a broad strategy, you can safely place your bets on these services. For the next year or two, the also-rans will be busy finding buyers and merger partners.

Now is when it really gets interesting, because now the action shifts to vertical market sites. For many marketers, this is where the more interesting opportunity lies. For example, in the area of health, there’s CarePages.com, Wellsphere, Patientslikeme, RevolutionHealth.com and iMedix. Seniors can choose from Elder Wisdom Circle, Grandparents.com, Eons, TeeBeeDee and Multiply. Mothers can sign up for Cafemom, MothersGroups.com, MomJunction and MothersClick, among others.

And the action isn’t limited to consumer markets. Sermo is a social network for physicians, which now boasts more than 50,000 members. Doctors exchange information about serious medical issues and review cases in real time. Pairup connects business travelers for peer advice, networking and assistance. There’s a list of more than 350 social networks here.

Don’t let small membership numbers fool you. Many of these sites may be attractive marketing venues. Scan the groups, discussion topics and participants and look for content profiles that match your market. Prices are generally lower than those of the big social networks and the audience is far more targeted.

Marketing to vertical communities is very different from mass marketing, of course. If you’re interested in building a campaign on Facebook, have a look at what Southwest Airlines and Victoria’s Secret are doing, or the group started by Starbucks fans that has over 60,000 members. There’s nothing particularly high tech about their presence. They mainly provide a place where customers can keep in touch with the brand and have access to special offers and downloads.

When marketing to vertical communities, you need to dig deeply into the expertise in your organization. Members of a health-oriented network, for example, want to speak to people who have lots of expertise in nutrition and treatment. Discounts and promotions won’t work nearly as well in narrow markets as they do in broad ones. If you have articulate, interesting domain experts in your organization, now’s the time to pull them out of the shadows and engage them with knowledgeable communities. Live chats, webcasts and Q&A forums are particularly effective.

Much of the media attention in the last year has focused on the battle for social network supremacy. With that competition now over, the market will subdivide itself in interesting ways. This process will continue for years, presenting an ever-shifting landscape of new marketing opportunities.