How often should companies post news into social media, send newsletters, etc. not to bother their audience too much? Basically, there is nothing like "too often." If you have something to say, say it. Big brands' communication is everywhere, you cannot escape, and that's why they are big.
As we have seen many times, relevancy is much more important than frequency. If you copy-paste something once monthly just to fill up your Facebook, rather save your time. On the other hand, we have customers who produce relevant personalized content several times a day, and their audience grows.