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Wikipedia doesn't consider Twitter a reliable source. Why should Facebook?

Facebook uses Facebook as a source, which is as reliable as Twitter.

Perhaps that obviates the need to include Twitter

Interesting question. I think that Facebook/Twitter and such are becoming utilities for a sizable chunk of the audience lately. So don't be surprised if they get to their Standard Oil moment in the future.

None, but it's important that people know that the trending topics are not what's popular on facebook, but rather what facebook employees have decided to promote on facebook.

Other than that, it's their platform and their company. They can run it however they want.

User Trust.

Also this isn't just about censoring Twitter as a source, but rather twitter as a topic in an trending topic.

Tangentially related: should facebook hide posts made by Twitter employees?