Social media marketing is how many are using social networking sites to make new friends and to discuss ideas and interests with other like minded people, so if you are a beginner and want to know where these websites are all about here is a list of some of the social networking sites I have come across:

CafeMom - is a community where moms get advice and support, connect with other moms.

Gather - makes it easy for you to meet people and make new friends

Meetup.com - Helps groups of people with shared interests plan meetings and form offline clubs in local communities around the world.

Yelp - User Reviews and Recommendations of Top Restaurants, Shopping, Nightlife, Entertainment, Services and More .

Bebo - Allows users to share photos with music, and blogs, and draw on members' White Boards

Blackplanet - The largest online community for African Americans, news, community, technology, culture, music, entertainment

Ning - An online service to create, customize, and share a social network

LiveJournal.com is a free service for all your journaling and blogging

LinkedIn is a networkig tool to strengthen and extens your existing network 

Perfspot- Find old friends. Have unlimited uploads and much more

Orkut - Social networking and discussion site operated by Google

Netlog is a youth community where users can keep in touch with and extend their social network

Zorpia - A free website to share unlimited photos, post journals and make friends

Friendster - is a leading global social network emphasizing genuine friendships and the discovery of new people through friends

eCrush -  is the place to find out if your secret admirer feels the same, without the risk of rejection; also quizzes, chat, love advice, match making

Dodgeball - is a social networking tool using mobile phones. It allows you to broadcast your location to other users, so they can meet up with you for social events.

Gaia Online - is an online hangout, incorporating social networking, forums, gaming and a virtual world.
Ravelry-

Box.net -  Online collaboration tools adopted by over 50000 companies globally.

Evernote - allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at any time, from anywhere.

myYearbook.com - a free interactive internet social networking service for highschools and colleges all over the world with videos, pictures, ways to contact others.

DirectMatches is the first online multi-match matchmaking system that helps people locate everything from business contacts to job opportunities and more.

Ecademy - is for online networking for business, offline networking events and global networking groups.

Care2 -  is largest online community empowering people to lead a healthy and green lifestyle while taking action on important causes such as human rightsXing -

Tweako - A user-powered site, and social network, serving guides, tutorials, and more in all categories of computing, technology, and the Internet.

Sphinn - is the place for web, online, search, interactive and internet marketers to share news stories, talk within subject-specificHubspot-

Wetpaint -  create a website built on the power of collaborative thinking that mix all the best features of wikis, blogs, forums and social networks into a rich, user-generated community.

Reunion.com - Now called MyLife™ helps you find people from anytime in your life, no matter where they are - all in one place.

TheFreelanceNation.com - is a network of 119 local freelance websites dedicated to helping freelancers effectively market themselves on the Internet.

AdGabber - is a social network for individuals in advertising, marketing and media.

Genkvetch - Social Networking Website for Baby Boomers and Beyond

If you are looking for social networking sites you will need to search for them by niche, see when a person joins a social site they want to be with their peers most of the time so when searching online just type in "social networking sites for kids, business, adults, tweens, women, professionals, moms, seniors, children, teachers, knitters, etc. you get the idea, be specific and you will be able to find the groups you want to be a part of. What are your hobbies, interests, goals, or experience, then look for groups that match those things.

I came across one of the best list of niche social networking sites by Kevin Palmer of Social Media Answers each is seperated by category such as:

• Age, Alumni, Art, Aviation, Beer, Blogging, Books, Brands, Business, Carpool, Business Travel, Cars, Auto, Motorbike, Cartoons, Charity, Comics, Cooking, Dating, DIY, Dolls, Education, Environment, Ethnicity, Events, Exclusive, Family Genealogy, Fashion, Female, Gambling, Games, Gaming, General, GLBT, Gothic, Food Beverage, Health and Hobbies, Instant Messaging, Interior Design, Jobs, Journal, Language, Legal Profession, Local, Media, Medical, Memorial, Military, Mom, Movies, Music, Nightlife, Outdoors, Pets, Photos, Political, Questions, Real Estate, Regional, Religion, Reviews, Science, Shopping, Smoking, Spanish, Sports, Student, Talent Showcase, Technology, Teen, Textbook Swap, Time Capsule, Timelines, Training, Travel, Video Games, Wine, Writing

Now I would say this is probably the best list you will find to date, so I recommend you check out his blog in order to get the actual social networking sites listed under each of the above categories.