I've been using Acarsd for a little while. I'm in the Atlanta area, and while at work I can get the uplinks on a couple of acars freqs. From monitoring several of the freqs around here, I've noticed the different uses of the freqs, and found a couple (I guess) "local use" acars freqs, for example, 136.575 is not listed as an acars freq anywhere but I have logged Delta and a couple other airlines passing terminal messages from Hartsfield. 131.55 is used mostly by the BizJets and smaller airlines in and out and around the Atlanta area, and by International airlines passing by. The uplinks on 131.55 show Arinc passing the aircraft to other freqs for messages, so I guess the acars network works like ATC's passing the aircraft to other controllers as they move thru the area.
136.8 and 136.85 are very active with Delta, UPS, Fedex, and other major carriers, 130.45 is fairly active with a good mix of carriers as well as 130.025. 129.125 is not as active, but still has a good mix of carriers.
Play around with the different acars freqs with what software you like using and see what uses the different freqs have in your area.