I started the hard way (by right clicking every person I saw in Mor Dohna, Dragonhead, and Costa del Sol with a different FC tag), then moved on to an easier method (several /searches limited to specific player classes, /sea against each of the capital cities. Threw in a few worldwide searches as well (200 players returned at max).

We haven't access to APIs to pull the data and Lodestone is a pain to mine. The data's meant to be a general stat sampling. If your FC was on and had at least 1 player in a searchable place, I probably saw ya. But maybe not! I ran into three FCs with 0 of whatever players on (the person I found on a search logged out before I checked their company info), and two FCs that were comprised of single members.

I figure the larger/more active FCs are likely represented, though.