Confirmation bias is a cognitive bias that causes us to see things in a way that confirms our preexisting beliefs. Because it involves selectively collecting and recalling information, the end result is a biased view point. In addition, confirmation bias may also cause us to ignore information that doesn't support our views. Closely related is the feeling of cognitive dissonance which refers to the discomfort we feel when holding contradictory beliefs.
In social media, we're often shown content that we're more likely to agree with. This amplifies confirmation bias--we only see things consistent with our beliefs resulting a sort of "echo chamber". For more on this, see filter bubbles.
Belief in farfetched conspiracy theories can often be a result of confirmation bias. If one tries hard enough, they can find things that seem to support the theory while ignoring the evidence that refutes it.