1. It's a lot of work for little return.
Oh yes, this is most definitely the number one reason you shouldn't be promoting your business on social media. To create quality content and engage with other people takes huge amounts of time. Time, you could be spending doing something else, like taking care of the customers who are already paying you.
This is a simple matter of opportunity cost. What things could you be doing that aren't social networking that will lead to better returns? That's a question you need to ask yourself every time you get ready to make a post, or engage with social media. For most business owners the best way to increase sales is to cultivate word of mouth referrals by treating existing customers like gold.
2. It's easy to lower productivity by getting distracted.
Even if you think it's worth your time to be on a social network there's a good chance your personal productivity is going down the drain. It's just too difficult to ignore the latest family controversy, political, or religious opinion. Those things are all fine and dandy when you have free time, but if you're on a social network promoting your business you need to stay focused. For most people that's a daunting task without all the distractions.
3. Followers are not quality leads.
The other day I saw someone post a question on a forum. "How do I get my followers to become customers.". The first thing that popped into my head was "Nothing.". Followers can't easily be made into customers because they're not leads. Before someone becomes your customer they'll most likely need to become your lead. To follow someone is such a minimal commitment and so un-targeted there's hardly any point in trying to get them to become leads much less take them all the way to being customers.
4. People aren't looking to buy when socializing.
If you're wondering why your followers aren't leads, this is the reason. People don't come to social networks to find things to buy, they come to socialize. Think about the last time you were on vacation or at an event and you're having a great time before being rudely interrupted by someone trying to sell you something. That's annoying.
5. Self promotion is viewed negatively.
Because it's annoying to be pitched on a product or service while you're trying to socialize, it's likely your attempts to promote your business on social networks will be viewed in a negative light. In fact, there are entire social networks like Reddit who's avid users pride themselves on weeding out those who try to promote themselves.