Listed below are some notable social networking sites. I've only included sites that have more of a professional networking focus (as opposed to dating or friendship). Some are very focused and others are more general.
-BusinessPartnerships.ca - Small or medium business owners looking to meet other business owners to interact and grow their business
-Contact Network - Contact Network finds important, but latent, business relationships that already exist within a company
-Doostang - Professional network that connects people through personal relationships and affiliations
-ecademy - Business exchange that connects people to knowledge, contacts, support and business
-eConozco - Online networking in Spanish
-EntreMate - The premier resource for finding business partners worldwide!
-Friendly Favors - People doing favors for each other
I'm Not From Here - For transplanted professionals
-ITmobE-network for IT professionals
-ItsNotWhatYouKnow - Business and career networking
-Kickstart-Connect - Brings together like-minded business-oriented and professional people in an informal and supportive environment
-Konnects - Serves as your personal business agent, by introducing you to new business contacts
-LinkedIn - Discover inside connections to recommended job candidates, industry experts and business partners
-Link Silicon Valley - Connect to the people who build and fund the technology companies of Silicon Valley
-matcheroo - Enables college alumni organizations to build various types of personal networks to connect people and groups
-Mediabistro - Connects media professionals to each other
-Monster Networking - Job related networking
-Netmodular Community - Blogging plus social networking
-Network PLUS - Global network of people who care about people, community, and business
-Networking For Professionals - Business networking
-OpenBC - Europe's leading business networking site
-PeopleThatClick - Connects current and aspiring entrepreneurs with potential business partners that offer complimentary skills, resources, contacts or capital
-PowerMingle - Online pre event matching forum for event delegates and exhibitors
-RealContacts - Helps you to find staff or jobs through your extended network of contacts
-ReferNet - Helps entrepreneurs find partners and referrals through online business networking
-Ryze - Business networking
-SelectMinds - Corporate social networking
Senior Management Network - Networking for senior management
-Sullivan Executive Networking Community - Online community for top tier executives to meet, network, exchange ideas
-Tacit - ActiveNet - Expertise sharing and relationship networking for the on-demand enterprise
-TENG - US Technology Executives Networking Group
-Visible Path - Leveraging relationship capital to drive revenue growth
-WisdomBuilder - Extract meaning from large quantities of information, all in an effort to promote innovation, productivity and profitability
-Zerodegrees - Connects you with influential people who can help your career, business, and personal life
Z-oomInfo - Database currently contains executives, managers and professionals in 1,538,217 companies - all instantly searchable to meet your needs