Facebook has over 1.5 billion active users worldwide. For many of them, the social network is their primary source of information - which can result in a limited selection that only reinforces pre-existing views.
Most Facebook users tend to network with like-minded people. Now experts are warning this could result in what they call a filter bubble - a limiting of content to only what reinforces the user's own pre-existing views. Other opinions and related information get filtered out - a consequence of Facebook's increasing function as the primary source of information on current events for many of its users. So they have little chance of forming well-rounded opinions.