Actually, all Social Media marketers, myself included get a lot 'wrong'. We're all learning every day, and without making mistakes we will never move forward.
But it's important to try and define 'wrong' and 'mistakes' because some people reading this article will say that some of the things listed work extremely well for them and the brand they work for.
For example, you could argue that putting focus on having a LOT of followers is actually a good thing and that for your particular brand it might indeed be the most important metric - ultimately it depends on your Social Media strategy.
Trial and error is arguably a strategy in its own right, but something that a lot of brands do get wrong is not having a Social Media strategy at all. And yes there are still many thousands of businesses (particularly those in the regulated space) that still don't have a plan.
One of the exciting things about Social Media is that it is important to include a certain amount of trial and error within your plan and then to learn from it.
What I do get from this excellent article (and links to other articles) by Jayson DeMers is the importance of keeping things simple wherever possible. Don't get too wrapped up in the business of Social Media (platforms, hashtags etc), but do get wrapped up in keeping clear focus on your plan and execute your activity accordingly.
It’s true that the organic visibility you can achieve on social media is shrinking—and has been shrinking for some time. However, that doesn’t mean that paid advertising is the only way to earn more visibility on your platforms of choice. Your best bet is to reduce the number of posts you make and instead focus on making better posts; posts with more likes, shares, and engagements will naturally earn more visibility, and a timeline full of only high-quality material makes your brand look better overall.