I don't actually think your situation is very different from that of others at large universities. I went to a very large place, one of the biggest in the US at the time. The math department was huge for the time with about 80 professors and twice that many doctoral students. But in the field I was working in there were only three professors in our working group. The fact that there were tons of topologists and algebraists in the department meant little to me. Research is specialized and normally done in small groups.
But, to actually answer your question, a facebook group for economics is probably going to be just what I describe above. Lots of people, but very few that have interests close to yours. What you need is a tailored approach not a broadcast one.
Your advisor or another faculty member in the department probably has already developed a circle of contacts and maybe collaborators. Ask to be introduced into such circles. See if the department can provide some funding either to invite a visitor to your institution for a few days or to send you on a visit to work for a bit with someone interesting.
Contact authors of recent papers that interest you. First just to say you are trying to join that field and value the paper, but also wondering if they would be willing to answer questions you might have in the future. First contact should be fairly light, so as not to overwhelm a potential contact. Or, better, get your advisor to introduce you and to recommend you to them as a future collaborator.
Go to whatever conferences in your field you can and introduce yourself to speakers there. Your goal is to establish contacts, not, specifically, to get help on your own projects. At a conference, take note of who asks questions of the speakers and introduce yourself to them. Ask questions yourself. Take a lot of notes with ideas about research ideas that look like they might be worth pursuing in the future. Associate those notes with the individuals that generated the idea in your mind.
As a faculty member, I had a wide circle of collaborators, but they were distributed worldwide - mostly US and Europe. No other faculty at my fairly small institution had the same interests. When I worked with a doctoral student it was either one on one or with the help of some colleague elsewhere than the university. That circle was developed over time as I've suggested above, but especially through conversations at conferences.