Learn French: Je pense que ce réseau social est utile. - I think this social network is useful.

Because this lesson focuses on the subjunctive with aimer que, you might expect the subjunctive here too! However, the affirmative penser que (to think that) introduces a statement you believe to be a fact. Therefore, it takes the standard indicative mood (est). We generally only use the subjunctive for emotions, doubts, or desires.

What happens if I make this sentence negative and say 'I do not think that...'?

Making it negative introduces doubt, which does trigger the subjunctive! If you change it to je ne pense pas que, the verb changes to soit: Je ne pense pas que ce réseau social soit utile.

Why is it ce réseau and not cet réseau?

Réseau is a masculine singular noun starting with a consonant, so it takes the standard masculine word for 'this' (ce). You only use cet for masculine nouns starting with a vowel or a silent h (like cet ordinateur).

How would I make ce réseau social plural?