Unless you are living under a rock for the past few decades, you’ve probably heard of Facebook, Twitter, MySpace, LinkedIn, and YouTube. They are part of an Internet phenomenon called social networks. The sites mentioned above are the kings of social networking. However, these sites have grown so big that users feel lost in them.

That is why there are niche social networks that are nipping the heels of these giants. A niche is a specialized subset of a larger set that targets a select segment of the general population. These social networking sites allow users to connect to people who have the same interests, professions, or hobbies as them.  Niche networks become great online communities that users sometimes share inside jokes, much like a group of friends would.

Most social networks earn money through paid advertising. Niche networks are a boon to an advertiser because it already has a relevant, ready-made target audience. For instance, a social network for pet lovers would provide a very effective ad slot for manufacturers of pet food. Advertising in niche networks have become so effective that companies spent USD280 million for advertising in these sites. In 2010, that number rose to USD1.9 billion.

Here are five of the most popular niche online communities in no particular order

1. Kaboodle

Established in 205, Kaboodle is a social network that appeals to people who love to shop. Founders of Kaboodle created the site to help people recommend, share, and discover products. But Kaboodle is not just any ordinary site because it does more than just present a list of products. In fact, it does not even sell anything. It provides tools to better organize a user’s shopping as well as find the best prices.

A person can set up his own profile once he registers on Kaboodle. He can then add products from any shopping website to his list. This feature eliminates the need to maintain and monitor several wish lists on different sites. In addition, each list a user maintains has a share option so other viewers can view the list. He can also compile style boards displaying his favorite looks and fashions. Add to that community polls, internal communication systems, blogs, and articles, and you would see why Kaboodle boasts of more than 12 million monthly visitors with more than 800,000 registered users.

2. Ravelry

Do you want to find people who are interested in knitting? Then go to Ravelry. This is a community site for knitters, crocheters, designers, dyers, spinners, and everyone who works with yarns, patterns, threads, stitches, and cloth. Since its introduction in 2005, the site is increasingly popular, and it currently has 350,565 registered users.

Ravelry provides users a venue to organize ongoing projects, listing patterns and materials being used, posting photos of finished works and works in progress, and commenting on others’ works.

Small-scale business that sell knitting and crocheting materials such as hand-dyed yarn and spinning fiber credit Ravelry for helping them advertise their wares.

3. Imeem

Audiophiles should open an account in Imeem, a free social network built around music. A user can interact with other music lovers, share playlists, and watch videos. A user can also build personalized playlists that they can share to the community. Members are welcome to browse and comment on everyone’s playlist.

Music on Imeem is streaming and free, however, you can still purchase songs from iTunes.

Independent singers, songwriters, bands, and other musicians should set up profiles on Imeem to share and promote their compositions. They can embed the music on blogs or in their profiles and post information like tour dates, interviews, and gig schedules. Presently, Imeem has expanded its features to attract independent film directors.

Currently funded by private investors, the site started in 2003 and currently enjoys traffic of 25 million visitors per month.

4. Classmates.com

Do you know where your high-school crush is working right now? Then you might find her in classmates.com, a website that connects current and former classmates. Launched in 1995, it is interestingly one of the earliest social networking being launched in the Net. Usage of the site is free, but if you intend to view other people’s profiles or send them a message, you need to purchase a “gold” membership.

In addition, Classmates.com has “interest groups” wherein users of one group can share information on a variety of topics. A message center feature allows users to communicate with each other without giving out sensitive information such as e-mail addresses.

Classmates.com currently has 4.6 million paid users and 40 million registered accounts. This is an extremely useful site for people trying to set up class reunions or finding out contact information for classmates that they have lost touch with.

5. Flixter

For movie fans, Flixter is the best online venue where they can get the latest information about movies (past, present, or coming soon), profiles of their favorite movie stars, fan clubs, celebrity gossips, movie news, video clips, trailers, and more.

Flixter members can rate movies, videos, and actors. They can also interact with other movie buffs through forums and other internal communication systems. Flixter also has a user-built wiki, which provides a lot of information about the movie industry. Users can add their own articles to the wiki.

Another great feature on Flixter is its integration with Facebook. Thus, Facebook users can add the Flixster application to their profiles. Facebook also shares any Flixter activity to the user’s feed. Finally, Flixter has an application for the iPhone, which allows users to visit the site wherever they are.

Flixter has more than 15 million monthly visitors providing 2 billion movie ratings. Private investors keep the site alive and well.

If you have a hobby, sport, business, or other interests, scour the Net. Chances are, there is a social niche for it.