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So you're back from vacation and culling through piles of e-mails. Don't be surprised if you find invitations to join the latest slew of social networking sites. There are a couple of trends emerging--social networking niches, if you will.

If you have business travel, you might try to connect with far-flung friends in even farther-flung places. This week, a new social network for business travelers announced funding. Called Dopplr.com, it'll inform your business contacts and friends of your travel schedule, so if you're crossing paths at a conference or looking for a good bar or steak in Santiago, you can poll your pals. There are already a bunch of social networks for travelers.

Asmallworld.net has a particularly international user base, and users try to meet up when on the go. There are a slew targeting frequent fliers--sites like FlierTalk.com have evolved from chat boards to real networks. And sites like World66.com allow users to share travel advice, creating a kind of wiki. But Dopplr.com is the first travel site aimed at the business set -- no surprise its investors include Reid Hoffman, the founder of business networking site LinkedIn.

Another social network niche--real estate--just because the bubble has burst doesn't mean people aren't still obsessed with their property. They just don't feel like going onto Zillow to check how much their house's value has dropped. Now sites like Propertyqube.com allow users to talk about their houses and neighborhoods. They can get in touch with experts and trade advice on everything from mortgages to real estate tax breaks. If you're an obsessive homeowner also check out Streetadvisor.com.

And then there's America's obsession with plotting family trees--I hear that genealogy is the nation's second favorite pastime after pornography. So if you're into hobby #2, check out Geni.com -it's pretty cool. You chart your family history, and you invite your family members to fill out branches further back on the family tree. It's a good way to aggregate disparate memories of great great aunts and uncles. And unlike sites like MySpace which attract a younger generation, this is nice and palatable for Baby Boomers.

This post is part of my new "What's Next" segment for CNBC's "Street Signs."

Questions?  Comments?