It is defined here what is on-topic on the AU Meta site. That currently being:
Meta Ask Ubuntu is the part of the site where users discuss the workings and policies of Ask Ubuntu rather than discussing Ubuntu itself. It is separated from the main Q&A to reduce noise there while providing a legitimate space for people to ask how and why this site works the way it does. Meta is for...
...Ask Ubuntu users to communicate with each other about Ask Ubuntu (asking questions about how the websites work, or about policies and community decisions)
...Ask Ubuntu users to communicate with Stack Overflow the company (posting bugs, suggesting improvements, or proposing new features), and
...Stack Overflow the company to communicate with the community (soliciting feedback on new ideas or features, or discussing policies that affect the whole network)
But recently I saw this which was posted a few years ago by @NathanOsman. Now as this does not match the rules above I would say that it is off-topic because Meta, as defined by those rules, is a place to post about the Ask Ubuntu website, as well as to communicate with Stack Overflow, and for Stack Overflow to communicate with us, but nowhere there does it mention anything about social announcements of leisure activities which may interest the community but have nothing to do with Ask Ubuntu, or Ubuntu, for that matter.
But after looking at the post, I see that it has been rather well received by normal users and moderators alike.
So if this is what the community wants to be on-topic, then it should be defined in the page that I linked which contains the description and what is allowed on the Ask Ubuntu Meta site, or that question should be closed as off-topic.
(The reason why I am bring this up now is because I understand that @NathanOsman is soon to post another one of these posts so it came to my attention and I thought I would ask on Meta about this seemingly off-topic question which has not been closed as such when the rules clearly currently state that it should be.)
Please note that if this is a question which is concidered on-topic, I am only proposing for the rules to be amended (because I don't see how "asking questions about how the websites work, or about policies and community decisions" covers these sorts of social announcements which have very little (if anything) to do with Ubuntu or Ask Ubuntu, and even if you are going to argue that the game is running on an Ubuntu server and thus it is on-topic, then how far does that go? Is a question on-topic about a picture of a Panda if I happen to take it with an Ubuntu phone?), however if this question is not on-topic, then it should be closed.
Clarification: The reason why I have put the feature-request is because I am saying that if the community wants social announcements such as this to be on-topic, then the help section should be edited, until then, it is very confusion what is on-topic and what is not because it doesn't clearly say that this is.