Is there an online community to find travel partners (for mountains, sightseeing etc.)?

Another possibility is meetup.com. Here in Vancouver, at least, there are several travel groups on it, and many of them schedule trips together.

You can also try the Lonely Planet Thorntree forums, and see who else is travelling in the same area, or even put up a post there for people to join you. It's fairly common to see that on there.

The best thing I can think of is posting in the local groups and events section on Couch Surfing. You can post your trek as an event and get others to join you or you can find events that others are hosting and join in. You don't have to host or be be hosted to participate.

I second meetup.com as a viable source though the quality of what you find in each city varies a lot.

There is also internations.org which is an expat organization which usually has groups in most major cities around the world. But you might need to become a paying member to access the activity groups.

And then there is a digital nomad group nomadlist.com which on the surface is a city comparison site, but if you sign up for a one-time cost, you can join the community on Slack which has subgroups for many cities.