The players who post on these forums are generally very active in-game as well. If you send a message to a character of Glaumring, Vellos, Valast, etc. you will probably receive a response. So cutting them off from the forums in order to force them use in-game messages isn't necessary, as they already use in-game messages.
As far as the lack of in-game messaging goes, I think it has more to do with people not being very active. I've been going around messaging Dukes about something, and you'd think that Dukes, being in important positions, would be some of the more active players. But apparently not, because the ones I messaged didn't reply back and when I looked at their character locations they never moved either. Also, when I was in Perdan there was some Duke who lost his position due to idleness every other week, but the ruler kept on reappointing him. It happened many times. So, if you're going to blame something for lack of messages, you should blame Dukes... or just inactivity I guess.
