Without wanting to cause any offence, because it isn't meant to, you remind me of a teacher of mine, Eleusinian
She had this habit of trying to get us to work on what she'd prepared. "It's fun," she'd say. "You'll enjoy it." This was especially the case with art activities. I was never good at art; I'm still not. Draw a horse and it looks like it should be barking. Just not my thing.
We were reading
Danny, the Champion of the World and she had us paint pictures based on the book. My "portrait" of Danny was... well, different. I can remember it well. I was 9 so it at least looked human. But his right leg was about four times as wide as his left leg. My teacher, bless her, put it up on the wall, just to show me how "fun" it had been. I remember the comment of one of my class mates who wanted to know if I'd painted Jake the Peg and if his extra leg was in the right leg of his trousers.
(If you don't know Jake the Peg, look it up. If you get something porn-esque back, sorry - I haven't tried

Diddle-iddle-iddle um.)
My point is that "fun" is subjective. I didn't find fun in art.
Danny was fun because it told a story of a totally different - and possibly no longer extant - way of life from that of a boy living in a distinctly urban district of a big city. So different that I expect, today, the book isn't read in classrooms. Seems that, today, anything which kids will have a problem empathising with, anything not easily identified with, is not part of teaching. For me, it was so different it captured my imagination.
I've no doubt that many of the kids in the class found art fun. It wasn't maths and it wasn't writing. It wasn't watching two hippies singing songs and playing a guitar while a big white dot bounced along words at the bottom of the screen, making us join in (which wasn't fun for anybody). Despite things not being fun for everyone, this "fun" was imposed upon us (and rightly so - this is school where we are rightly exposed to different experiences) and we were expected to find it "fun" because we were
told, in no uncertain terms, it was such.
Dip
is a social game. If you can't be bothered to communicate, you're playing the wrong game. And if you deliberately choose to not communicate, or not reply to a given player or message, there is a cost. I suggest that the cost is
strategic rather than social.
There are assumptions being made and, in my opinion, lazy ones. First, that not replying to a message is anti-social; second, that not replying to a message is - objectively - "non-fun".
I could reply to almost every message I get in a game. Does that make me social? Not necessarily - I could be an objectionable jerk most of the time. But, assuming I'm not (at least when playing the game; I may well be an objectionable jerk outside the game) and I respond appropriately to everyone, does my
choosing to not respond to one player make me anti-social? More specifically, is the single act of not responding to a message an anti-social act?
What I'm saying is that, when that choice is made based on strategy, then the choice is
about strategy; if the choice is about strategy, labelling it as being anti-social misses the point. It doesn't matter if the non-response is to a single message or to one player in a thread of messages: applying a generalised label to something - anti-social or against etiquette - is prejudiced; it is presumptive and, if the choice
is made strategically, inappropriate.
This is a game which was designed to be anti-social to some extent. What is more anti-social than betrayal? It's a game which probably involves lying or at least being misleading. How social is that? It's part of the game.
I recognise that communicating is also part of the game. When playing f2f it is difficult to ignore someone. But this game isn't f2f; there are different options open to players. One of those options is being able to not reply to a message. Possibly not the best option - but an option, nonetheless. It's a choice that is open to players.
I would also suggest that one player's idea of what makes the game fun is not, necessarily, another person's idea of what makes it fun. Personally I think everything about the game is fun. And that includes the challenges of working out what is going on.
If I don't get a reply to a message, I have to work out what is going on. Is the player gone from the game? Is he pissed at me (then, why)? Is it that he chose to not reply? Is it simply that he's not had the chance to reply yet? Can I get any clues from the board? What does his history as a player tell me? Then again, if the player replies to a message, I have the same challenge for different reasons. The point is that a non-reply is a challenge. For me, that's part of the fun.
Obviously, that isn't the case for other people - well, the challenge might be but not receiving a reply to a message is more of an insult than anything else. OK, that's fair enough. But is it my responsibility to ensure that I please everyone in a game?
I think that playing any game carries responsibilities. As I said before, there are some aspects of playing this game that can be classified as "etiquette". I think they apply to playing any game. One of them is that you need to remember that other players want to join the game, too. The question is how far you take that.
Do I have to make allowances for everything that might possibly prevent someone from enjoying playing Dip? How far do I take that? Do I have to reply to messages just in case I'll upset someone by not doing so? Perhaps - if not receiving a reply would upset me.
In another topic one reason given for not breaking an alliance to try to win the game was expressed as:
If that final stab doesn't seem fair to me, by which I mean the victim will be personally insulted because they can't see the game-motivated reason for it, then it just feels wrong and undermines the part of this game that I enjoy [the interaction between players].
The stress is mine.
According to this view, maintaining an alliance is something to do if the player who you might otherwise stab may not be able to see the reason for the stab. That is a point of view; not mine. The inability of another player to understand my reasons for stabbing him isn't my problem.
In the same way, if another player can't appreciate any
strategic choice I make, including - potentially - not replying to a message, that isn't my problem. And the only reason for not replying to a message, IMO, is a strategic one.
I have to say, I can't remember a time when I
haven't replied to a message deliberately - at least, as part of a longer strategic discussion. That is mainly because I think that it makes more sense, in most situations, to reply and be evasive than to not reply

But I certainly wouldn't dismiss it as a strategic option nor on grounds of it being "non-fun" or anti-social. It isn't up to me to define what fun is for other players... but neither would I go out of my way to spoil the game for other players.