Learn Russian: Просматривая социальные сети, мы теряем продуктивность. - Scanning social networks, we lose productivity.

The golden rule for using verbal adverbs (деепричастия) is that the action of the verbal adverb and the action of the main verb must be performed by the exact same subject. Because мы (we) are both scanning social media and losing productivity, the verbal adverb works perfectly. If the subjects were different, you would be forced to use a clause with когда (when) or пока (while).

How does the form of просматривая show that the actions are happening at the same time?

Просматривая is an imperfective verbal adverb, formed by taking the present tense stem of the imperfective verb просматривать and adding -я. Imperfective verbal adverbs always describe a background action that occurs simultaneously with the main verb (теряем).

Is the comma after сети strictly necessary?

Yes! Unlike in English, where short introductory phrases might not need punctuation, Russian grammar strictly requires verbal adverb phrases (деепричастные обороты) to be separated from the rest of the sentence by commas, regardless of where they are placed.

Why does социальные сети take the accusative case after a verbal adverb?

Verbal adverbs act exactly like their parent verbs when it comes to governing objects. Just as the verb просматривать (to scan/look through) requires its direct object to be in the accusative case, its verbal adverb form просматривая demands the same case.