Is there a "Facebook" for researchers? (to collaborate on projects) I'm writing my PhD-thesis in economics at a fairly small university.
Accordingly, there are relatively few scientists in my department.
I am planning to write a research paper on a specific topic that nobody else at my university is researching.
However, I think that it makes sense to write this paper with several people so that the expertise of others can also be included in this study (and many more reasons).

My question now is where I can find other researchers on this topic. Is there a "Facebook" for researchers? And how can I contact them?

I would also be pleased if someone would tell me about their experience on this problem.
Facebook is not a collaboration tool. It is a tool for advertising companies and the government to gather information about people; that is the business model and the primary intent. Interaction and collaboration are at best secondary effects.

Now, other social networks controlled by commercial entities and many have business models which involve advertising, but Facebook is among the worst.

At any rate, have a look at:


https://researchgate.net
https://academia.edu 
https://linkedin.com


The first two are sites intended for researchers to showcase their work using a generic UI/UX. Users of both complain about their spamminess; and Academia.edu apparently has most functionality limited by a paywall. I personally have found ResearchGate less spammy than Academia.edu, but some comments below suggest otherwise.

The third one, owned by Microsoft, is more geared towards recruitment/head-hunting, and is about presenting one's professional profile rather than research. But I've found both ResearchGate and LinkedIn to be places where I've discovered people who are potential collaborators or sources of useful material for my research. I have not, however, used these networks to actually engage on collaboration.

But of course, that is also true for scientific conferences and journals, or rather their on-line form of websites and downloadable proceedings.