The forums have been nuked because it's too easy for people to search for complaints that way.
If staff tried to delete complaints on the forums, those that frequent the forum would complain that they were being censored and would cry foul. One of the main reasons that the SakuraCon forum shut down was because an attendee did complain about the convention, including staff names, and it seriously irked the management. The same could be said with Anime Expo; It was very easy to find threads regarding complaints.
With Facebook and Twitter, these topics can be quickly moved off the front page. Most people don't hit "See More", and if they do, they might only be willing to scroll down a couple of posts. It just boils down to convenience. If the answer is easy to find, then more people will know about it. If it's not and requires a Google search and the reading of several websites, most people won't. People often believe the source that's easiest to reach, even if that source is incorrect. Case in point - The idiots clamouring for CLAMP to return to AX in 2016. If they had bothered to have Googled CLAMP's prior visit, they would have noticed that CLAMP were total douches, going as so far to make fans stand several feet behind the table to present a pre-printed shikishi (the only item CLAMP would sign). They also cancelled their panel to go shopping, and when staff complained, they said "We're CLAMP, F you". So, yes, anyone who bothered to look this up would not be requesting them.