I'd say because of the nature of pop-ups you're really relying on industry relationships. All social media outlets are obvious places to start but that's friends and family mostly. Next I would advertise to any other chefs, owners, purveyors, writers or critics you've worked with and respect/like. If the pop up is in a business location (perhaps a restaurant closed on that day) than since they've already offered the space they're usually willing to accommodate some sort of signage or poster/flyer/what-have-you for at least the week leading up to the event. Filling these seats can be hard but since you already seem to be progressing with the concept than I'm sure you probably already have a solid network encouraging your abilities. The people who usually come are those who know and love your food from previous experience. As you do more you should establish a specific brand that you can focus on advertising through so as you (hopefully) gain acclaim more people will want to bring fresh faces to experience what you have who in turn will do the same for the next event- I.e. Food trucks tweeting their next location. I did pop ups and private chef gigs here and there but it's def not sustainable, the best advice I can give is to just take advantage and enjoy the opportunity to be creative. It's a blast if it's something you do on a day off with a foodie or chef friend but a bit stressful if you have any real goals of profiting greatly from the endeavor in any way other than growing your name. Good luck!
Social media and word of mouth are prolly your best contacts.
After all (like alaminute pointed out) popups are mostly just one offs to have some fun with your fans and friends.
Would be ridiculously difficult to make a living out of it.
The commissary fees alone would eat up any real profit.