I'm going to take a guess in that they speak to a lot of fans on show day meet n greets also through being private messaged on there own social media. They might lurk from time to time though, but they are not ignoring fans.
Axl himself, is an A-list celeb, so anything he says is likely to be published to a wider audience, than say a quote from any other member of the band, so perhaps he is more careful when he chooses to speak online. I also think he is just not into social media, might be a generation thing... Or he uses a fake account to keep into contact with his friends online. Do any other artists often participate in fan forums? It's easy being GNR fans to say "why don't they post..." but it might just not be the done thing for celebs.
He's probably into social media, but it doesn't involve GNR. The band collectively busted their asses to promote when they started out, and talking about GNR is promotion-related. If there's nothing to promote, there's nothing to say. It's no different than any rock legend who might show up on social media once in a great while to do a Q&A.
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