Websites for finding travel friends (with most members)?

The main problem is these days, almost every big site tries to bill themselves as a community. In addition, many of them don't release stats on demographics, membership numbers and so on.

Some of the most popular ones include:

Couchsurfing - over 5.5 million members in 97,000 towns, in over 250 states or countries.

TripAdvisor claims to be the largest travel site in the world, but much of it is reviews, but there's over 60 million members.

Frankly, if you're adventurous, I'd suggest smaller community-oriented sites. If you're a cyclist, WarmShowers (35,000+ members) has made many of my cyclist traveller friends meet people, and frankly I'd be remiss if I didn't mention our own site - Travel Stack Exchange, where we currently have over 16,000 users - accumulated in under 4 years. Many of us have met in person, and there's been at least a couple of meetups as well.

Speaking of which, Meetup is also great, if you specify 'travel' in your interests, you can find travellers nearby. I'm going to one this week!

I think Couchsurfing will be the best for that, they have a lot of forums and community groups. For sure you'll find travel buddies:)