A classic example of this was the announcement earlier this year that 70% of consumers "said mobile advertising is a welcomed personal invitation from brands, rather than an invasion." Of course, whether people think mobile advertising is a "personal invitation" or a "personal invasion" isn't the pertinent question; the appropriate question is whether people trust mobile ads and whether those ads convert mobile surfers into mobile shoppers and buyers. On those more vital questions, other studies tell a different story--Millward Brown recently found that consumer favorability toward mobile ads was so poor that it ranks with non-opt-in email, and Nielsen found that trust in mobile ads was lower than every other ad medium. Ask the wrong question and you will get the wrong answer.
I would suggest that these two answers are one in the same. If people received what they expected to receive from the brands they follow, it would be difficult for the number of brands' posts to rise to the level of "too much." Follow Mashable on Facebook, for example, and you will get a couple dozen posts a day, but Mashable has a million fans and almost 50,000 people "talking about this," so clearly they are getting something right even though they break every "rule of thumb" for frequency of posts. They offer valuable content people welcome, and so long as they do that, it is hard for Mashable to fall on the wrong side of the "too much" perception barrier. And this isn't just the case for media brands, either--how often could Disney or Harley-Davidson post before people would cry "too much"?
Ask the wrong question and you will get the wrong answer. If you ask people whether they unfollow brands for posting too much, they will answer in the affirmative. But the psychology behind the decision to unfollow is not really about quantity of posts but their value. If consumers saw more value, they would welcome more posts.
This isn't to suggest your brand has carte blanche to post as often as it wishes but to advise you ignore studies that ask the wrong question and instead focus on the needs and expectations of your own audience. If you bring laser focus to how your brand can truly and selflessly serve those needs and expectations, you can pretty much ignore all those studies and "best practices."
No study can tell you how often your audience will accept your brand posting in social media, but your audience can. And that is the right question to ask!