The estimates in sales I am seeing are that Aliens Defiance #1 was the top Dark Horse seller in April with sales just over 16,000. In May Defiance # 2 sold just over 11,000 and was the the number four top seller for Dark Horse. Keep in mind, May was heavy in competition as we saw the launch of Civil War # 0 along with DC Universe Rebirth # 1, Punisher # 1, Captain America Steve Rogers # 1 and a new Deadpool series. Competition was crazy for Dark Horse and Aliens so comic readers had a lot to go after last month.
Looking at Fire and Stone Aliens numbers it sold just under 20,000 for issue one and didn't dip below 13,000 until issue four in comparison.
As for the question is it selling well? Aliens Defiance sold 16,000 in April in comparison (if that is even considered fair) Black Panther # 1, the top seller of the month, sold 253,000, Star Wars Poe Dameron # 1 sold 175,000 and was the second highest and Dark Knight III Master Race sold 143,000 and placed third. But Aliens did outsell well known series like James Bond and Back to the Future along with Injustice to name a few. So personally, I believe the numbers are around where Dark Horse projected them to be.