Is Mindfulness the Opposite of Multitasking?
Mindfulness(Sati) and one-pointedness(Samadhi) are 2 different things. Technically speaking, the mind cannot take multiple objects at once. So there is no such thing as multitasking with the mind. You feel like you are multitasking because the mind keeps switching so fast. So it is possible to be mindful while you are on so called 'multitasking'. 

On the other hand, in the realm of mindfulness, the internet does not exist! Social media does not exist! 'I' does not exist! There are only four types of mindfulness: mindfulness of the body, mindfulness of sensations, mindfulness of mental activities and mindfulness of phenomena. So be careful of what you consider as 'mindfulness'.