I tend to avoid bashing Discourse's theme directly, as themes tend to be a matter of personal taste although, I also don't like it very much. It feels like they're trying too hard to be different.
Another point which people commonly use against social networks is privacy. Honestly, thinking that your information is private on the internet is silly to begin with especially, in a public place and I really doubt that the FB Staff or the NSA really care about your specific activity.
Honestly, if people didn't care about the privacy scandals enough to leave before then, I don't see how they will leave now or in the future over it.
As for forums, it's not too different. Any forum owner can just look in the database to view people's PMs, while Discourse has just
dropped any illusions of privacy by making it a core feature for admins (Discourse admins are superadmins, as they have no concept of admin permissions).
Personally, I don't really care about what users discuss over PMs, as long as they aren't breaking the rules (spamming, breaking the law, insulting people, etc.) and so, I don't really make an active effort of actively moderating what they're doing on there.
One thing which social networks tend to have against them is poor moderation. Forums for the most part, tend to be very well moderated, with some exceptions, while social networks more or less let users do just about anything they want.
Sure, there's moderators (unless, it's Google) however, I doubt that they'll do anything unless, it's really serious.
