A trademark grants you exclusive use of your mark as well as protection from competitors marks that are confusingly similar. Also trademarks can be lost if not

A trademark grants you exclusive use of your mark as well as protection from competitors marks that are confusingly similar. Also trademarks can be lost if not defended, unlike patents and copyright. The dispute between teachbook and facebook likely revolves around being confusingly similar, and could also be in part because they need to actively defend the trademark in order for it to be valid.