It is a very broad categorization to call anyone in-between a middleman. By that logic, these are all middleman because I use their service to sell a house:

1) My ISP because I use internet through them

2) My phone service provider, because I make calls via their network

3) My car manufacturer / leasing because I pay a monthly fee to go visit the listing

But, by my perhaps opinionated definition, none of the above should be classified as active middlemen because they don't interfere with my transaction w.r.t the listing. Facebook and WhatsApp are not active middlemen. They are simply just a listing service. I could replace them with say, Craigslist or even a Google sites web page and I would still be fine. The worst that could happen is I might be asked to pay a small fixed fee like $20 for a listing/webpage. The service provider (generally) doesn't care what the listing is about. That's why it's passive.

Real estate agents are active middlemen. They in most cases prevent the transaction altogether if you don't use them. They are not asking a fixed fee, they are asking for a percentage of the transaction - when the value they add doesn't compound with the transaction amount. That's why.