How to motivate visitors to comment
I would think the simplest option would be to contact barber/salon owners and inform them that your site exists, that their business is listed, and that as an owner, they have the opportunity to have their satisfied customers post a positive comment on your site to increase their customer base.
Bare in mind that you will need to make this really simple for them- something like a code to enter that will take them to their profile instead of a complicated URL. As well, you can offer the business owner the opportunity to comment on their own business to better define any unique value proposition (marketing speak) they may have such as discounts for service members and so on.
Also, know this. I have a friend that has a barber shop, sadly- I no longer need a barber, but he gets a ridiculous number of calls trying to sell him something, telling him about online opportunities such as business listings, and so on each and every day. It will be hard to crack that nut. But I suspect a friendly e-mail or letter would go really far. Emphasis the win-win opportunity for both of you. Gentleness/kindness encourages people to participate as a kindness back to you, however, business speak will not.
You can also make it easier with a simple rating system for those who are reluctant to comment. You can target a potion of your PPC ads and offer a coupon/rebate of anyone who leaves a comment. Something like: Love your barber? Tell us!
I am sure there are many other ideas. These are just a few. Good Luck!
Here is a fun idea! How about hosting a humorous haircut contest above or in your comment section. You can offer a cash reward for the funniest haircut- you can split the reward to the poor soul wearing the unfortunate haircut and the barber/salon owner who can cut it gain (if they want) with any employee. Site visitors can vote on the haircut and the highest vote count wins. Of course you will have to regulate voting a bit with one vote per IP address, CAPTCHA, and an other check to prevent gaming. Alternatively, you can use the comment section for votes too! This is an opportunity for actual barber/salon customers to participate. You can encourage commenters to not only comment on the haircut, but their experiences with the barber/salon too. It is too late for me to participate, but it could work for others! As well, you might end up with some unique content that may attract new users. Who knows?
I took a course back in the day on web marketing however I am no pro in that area. My opinion is, I think social media is hard to compete with left or right and I'm betting that's what is happening to you. My best answer is to create an attraction to the comment section of your site, you have to get your name out there.
As one person said, advertise, physically! Talk to the Baber shops see if you can leave a pamphlet or posters or physical posters that attract the eye. Ask the barber shop to hand these so so they can boost their rating.
While doing the above could bring in more people and make them aware of your site it will not necessarily make them want to comment. However, attracting people to the comment section would be your next step, maybe a poll, a flashing link at the top, "Check out the new poll!" The poll is attached to a comment section. In all, your ideas to drag your audience to the comment section where the poll would be. If you do not like the poll idea, try to start a conversation in the comment section yourself.
It comes down to "physical advertising", "attraction" and "pointing to" on the webpage. I cannot say for sure it will work but in the end, it could still attract a bigger audience.
I hope this helps you and good luck.
Edit: have you thought about a rewarding an audience member with maybe free haircuts for commenting, just another idea.