I don't know about Facebook. However I was recently perusing some reviews and the most common complaints I've seen against AH are:
1) Poor Steam performance. Even within the past few months there seems to be a large number of people having problems getting the game to run properly under Steam, whether performance-wise or just plain not working at all. Given that Steam is now a cornerstone for gaming content delivery for many PC players, I can see this as a significant problem for growing the player base.
2) The subscription. The complaint I see that's turning people off isn't that there's a subscription in of itself — many are more than willing to pay one — the problem is the cost vs. other subscription-based MMOs. With Aces High being twice or more the cost of other subscription-based online games this is seems to be a very common reason for players to pass it up in favor of F2P like War Thunder (even in spite of WT's Pay-To-Win aspects). ESPECIALLY when combined with the steep learning curve, as it's not uncommon to spend your full two week trial playing every day yet STILL struggle to be in any way effective. Some people simply don't play enough to justify the cost (this, unfortunately, is the reason that I've let my subscription lapse; I simply have had far too many things calling for my attention to have time to jump online for a couple hours every night).
3) Dated visuals. While the most recent models like the Wildcat (still wish we got a proper -3, tho) are without a doubt stunning, it only makes the older ones still in the game stand out more. Even aircraft that have already received updates like the P-51 and Ki-84 are looking long in the tooth. The simple reality is that other than the Retro Gaming niche, shiny top-end graphics are a major draw, especially for flight simulators.
1 and 2 are probably the easiest to find solutions for. 3 is a bit more problematic, as any major update to the graphics engine not only takes time, but risks putting the requirements beyond what some players may be able to afford.