This is pretty much true of any online community built around the opposition to something. People that had a positive or simply ‘meh’ experience don’t tend to spend time talking about it online.

The r/exmormon subreddit has 155,000 members. The r/exchristian subreddit has 60,000 members. When you compare as a percentage of global population of members of their respective religions, it looks even larger.

You're right that people who have had "meh" experiences don't tend to talk about it online. What is it about being mormon that makes exmormons so bitter, in comparison to more benign religions? Try reading the subreddit and you'll find out.

I'm not a Mormon, admittedly know very little about the religion, and don't want to make any statements criticizing or defending it. But it is a mistake to assume that the size of a subreddit is anything but a tiny representation of the real phenomenon. Reddit is not reality. The overwhelming majority of people don't use it or even know what it is.

It may simply be the case that Mormons are more tech-savvy as a group than Christians as a whole and thus they gravitate to online communities.

That's true, it's very hard to conclude anything about a religion by subreddit size, but it is actually pretty simple to infer it.

The largest ex religion communities online (former scientologists, mormons, JWs, adventists, and muslims, in my experience) all have a few things in common. They are from religions with significant control over your life when you are a member, and significant pain and anguish when you decide to leave. For some, that may mean losing your family and friends and professional colleagues (as I have). For others (such as scientologists), that may mean lots of harassment and legal difficulties. For others (such as muslims), that may mean being put to death.

I have no qualms with calling /r/exmormon a very bitter community. We're bitter...with lots of reason to be so. This isn't anything like giving bad reviews on twitter...this is a very active community of thousands of people who have had very negative impacts on their lives due to their association with a religion that controlled large parts of it. They're trying to cope in the only way they know how, by commiserating.

I would say that that should be a very valuable source of information to anybody who is investigating a religion. Sure, if you ask a question, you're going to get very biased answers. But they aren't one off problems, and they aren't dismissible simply because they're bitter.