I’ll say it again. Facebook is a very vulnerable company if it doesn’t clean up its act.

Last Friday, the stock of the social media company dropped by nearly 20 percent, causing investors — including co-founder and Chief Executive Mark Zuckerberg — to lose more than $120 billion, the largest single-day decline in value ever.

The reason: Facebook announced that daily active users in the US and Canada, its most important markets, have flatlined at around 185 million.

In Europe, it’s even worse. Daily active users there declined in the second quarter.

Plus, overall advertising revenue missed analysts’ growth projections.

True, Facebook is still a very valuable company, with a market cap at just under $500 billion. But a record-setting drop in value proves what I’ve been saying all along — that Facebook and its executives had better pay more attention to the details of their business or risk not only losing users but also alienating advertisers who may already be wondering if being on that service does more harm than good.

As my longtime readers know, I’ve been on Facebook’s case for allowing pedophiles to post their filth without much consequence. My crusade has been going on for years, and there’s no sign that Facebook is making much headway against these pervs.

And I understand why. Facebook’s site is just too massive to be thoroughly policed. And even when something naughty is spotted, Facebook’s policing techniques are just too lackadaisical.

Advertisers can’t tolerate that, so ad revenue is likely to come under pressure for that reason alone.

There are also the much more widely publicized issues of fake news on Facebook and the company’s carelessness with users’ data.

Put all of this together and it wouldn’t be surprising if Facebook is much smaller 10 years from now. And just the prospect of such an outcome makes this company and its stock as vulnerable as it looked last Friday.