Yelp is primarily used as a tool for assessing the quality and/or locate the services of a business that satisfy immediate needs; restaurants, bars, coffee shops, gas stations, shopping centers, lodging, and emergency services. Many wouldn't think to search for a Realtor on Yelp, but they are there; brokerages too. In order to set up your free account, simply search for Yelp for Business Owners, and they will be able to set you up. Though you may have to wait around 24 hours for them to verify your business.
Facebook is a social media platform and great for developing awareness of your business. You don't need to create a whole new account for your business as you are able to create pages from within your existing account. Just like any other business account on Google and Yelp, you will be able to set descriptions, hours, locations, services, and add photos and videos in order to generate flow to your page.
Outside of the occasional person searching for realty services on Yelp and Facebook, there is a secondary, and possibly more beneficial use. The primary benefit of Yelp and Facebook is that, in the same way that Google uses Google Reviews, Bing uses Yelp and Facebook. Based on research on search engine use, Google dominates at around 74% of desktop searches and 90% of mobile device searches; while Bing hovers around less than 10% on desktop and 1% on mobile. It may seem that putting your effort into Google Reviews would be better, and it may very well be, but since everyone is focusing on Google you can also work the Yelp and Facebook angle to garner those small percentages in Bing; less competition means more business from this small pool of internet denizens.
Also for ease of staying mobile, as with Google My Business having its own app, Yelp also has its own app for business owners to manage their profiles separate of the regular Yelp app your clients would be using on their smartphones. For your Facebook business page, you simply manage it straight from the same Facebook application you manage your own personal social profile. Google Review, Yelp, and Facebook have the means to allow you to manage your business reviews from your phone, as well as your desktop. Aww, 21st Century!