...in some areas. If you are selling sewerage treatment filters or unblock toilet for a living I doubt social media is ever going to be a useful marketing tool.
And social media usage is changing, mainly because of people using it for marketing. Lots of people are moving onto other platforms because they want to get away from the constant promotions.
If you are a local butcher you can use FB to lett existing customers about weekly deals or whatever. But if you sell cheap socks I doubt anyone is going to share or like your 5 for £2.99 deal.
I agree that in that instance you'd have a hard time. But your strategy is wrong from the get go.
On the first point, it's hard to say that any particular demographic isn't using social media now. It was a thing for techies and teenagers ten years ago, but that's not the case today.
Your nan is as likely to use it as your daughter.
The people who buy sewerage treatment filters are on Facebook the same as people with toilets (everyone?). There are some definite trends either way for certain demographics (more women on Pinterest, more teenagers on Snapchat) so you might be best to pick and choose your medium to get the best from your efforts (same as any marketing).
The fundamental flaw in your treatment of social media is the supposition that you need to be talking about offers, deals and sales - who would want to know somebody like that? If you sat down the pub telling your mates about your offers and deals all night they'd soon tell you to drink somewhere else. Sure you have to sell something eventually, but if that's ALL you try and do, forget it - you'd be better off not bothering.
This is exactly why it's not a short term solution. It's a way to reach people, highly targeted to your proposition, at scale. It's up to you how you treat them when you get there.
Execution, not medium is why most people fail at ANY form of marketing - social is no different.