How often do you get emails, or how often do they post on their website about the latest new members, upcoming events, and how much money other businesses have made through bartering. Try to find a barter exchange that constantly brings in new members and puts on fun events like a Christmas party, networking events, or luncheons for all of its members.
You'll probably have the most success finding new customers for your business by meeting other small-business owners in person at local networking; how many bartering networks in your city are actively bringing in new members every month.