I personally don't believe every single post needs to be read. This is my own opinions of course others need to disagree. But when two people are talking to each other and it doesn't involve the story, or the character you're a part of whatsoever then you have no obligation to read it. You may want to it may help solidify your understanding of other characters but it can also lead to some meta gaming potential say what if character A said to B that they liked cake. Then when character A meets C then C offers the aforementioned Cake for no reason. 

Basically it's up to you in my own opinion. I feel the need to add 'my opinion' because I don't feel like arguing about this.