As a user, rather than a business or advertiser, it annoys me intensely when businesses use Facebook as an advertisement platform. Sure, I may have "liked" your page, but that doesn't mean I want my feed to be bombarded with promotional rubbish. You've only got a very thin line to tread before I block you completely. If you want to genuinely engage with your "likers" then that's one thing, but just telling me to buy your stuff all the time is not what Facebook is for.
Twitter is a bit different – there's no reason to follow someone on Twitter if you don't want to receive their messages, so if I've followed you then there's more of a chance that I'm interested in hearing what you've got to say. But even then remember that to an extent you're preaching to the choir. You don't need to convince me that your business is a good one, I know that already. I don't want generic adverts. If you've got a special offer on, if you've got a new development coming out, that's the kind of thing I might be interested in. But more of the "same old, same old"? Forget it.
If you want to know what's acceptable and sensible practice, think about what works on you, when you use Facebook and Twitter. What tactics are more likely to get you engaged, and what turns you off completely? That's where a lot of businesses go wrong with social media – they see it simply as a way to push their advertising onto people, without considering how they would feel about other companies doing the same to them.