1. TripAdvisor – “Featuring real advice from real travelers, TripAdvisor-branded sites cover 300,000+ hotels and attractions.” The largest travel social network out there. Add your own reviews and share with 6 million other members throughout the TripAdvisor network.
2. Yahoo Travel – partnering with FareChase, Yahoo Travel lets you plan your trip in detail and share it with others. Yahoo Travel features reviews, peer advice and plenty of other features for any type of traveler. Use it to list your own trips and offer advice to other travelers.
3. Flickr travel photos. Although Flickr isn’t travel specific, it’s an excellent place to market yourself, if you have pictures available. Post photos of your travels (or just your own city) on Flickr and comment on other people’s travel pictures.
   photo credit: Premshree Pillai
4. Travel Daily News. Not social networking per se, but a good resource on travel news that lets you comment on the articles. Use it where it’s relevant – add your own qualified take on the news published. Another source for travel news is EyeForTravel.
5. Dopplr – a social network for frequent travellers. Anytime you travel, update your Dopplr trips. Connect with as many people as possible and share your trips.
6. Momondo – the flight search aggregator is growing, now including trip advice, destination reviews and a social network for travelers. Watch this space, Momondo will probably be a global competitor soon.
7. Pairup.com connects business travelers and lets them share travel plans and search for business connections at their next destination. A good resource if you are targeting business travelers.
8. CityTherapy is a social planning tool for European city life and city travel.
9. CouchSurfing – this non-profit social network connects people and place internationally, to create educational exchanges, raise collective consciousness, spread tolerance, and facilitate cultural understanding. A place for managing your reputation and getting engaged in with travelers with a conscience.
10. iloho is a social network that lets its members share advice and experiences on places around the world.
11. MyTripBook is a social sharing site for reviews and diaries on places members have visited, finding travelmates or information about a destination.
    photo credit: doragonet
12. Matador focuses on travel writing, and lets you become a paid travel writer. You can get info from experts, meet people at you destination or get inspired by travel articles.
13. Rummble is a location based content discovery tool, integrating with a number of social networking applications like Facebook, YouTube, Twitter and Wordpress.
14. TravBuddy is an online community for meeting travelers and sharing travel reviews, photos and blogs.
15. TravellersPoint lets you build your own online trip diary and connect to travelling friends.
16. TravelTogether lets you share itineraries, prices, pictures, options, and other details about travel.
17. Tribe.net is not a travel site, but lets you connect to people to get things done in your community. It is used by travelers to find local information and connect to local people at a destination.
18. Triporama is a trip planner and travel guide for groups. If you are focused on group travel, get engaged in this community and share your travel tips.
19. VibeAgent.com lets travelers research and book hotel accommodations. It combines user-generated hotel reviews, meta-search, and social networking to provide its users with free personalized hotel recommendations and booking at the best available rates.
20. WAYN – A social networking website uniting world wide travelers.
21. WeExplore.net provides an online community for international volunteers and other alternative travelers.
22. Wikitravel is a project to create a free and reliable world-wide travel guide. Get your travel website(s) listed here and contribute to the community with dependable advice.
23. Tourism Internet Marketing is a blog for travel marketers to get engaged in, that includes a number of tips on travel marketing.
24. Woophy stands for WOrld Of PHotographY. It is focused on making Internet navigation more visual, logical and associative.
25. PassportStamp.com, a site for tracking your travels.

Of course, this is not an all-inclusive list. I will add new top 25 lists to this in the future.

What is missing from this list? Are there any travel specific social networks or applications you think should be on this list?