“Well you can't believe everything you read. After all, by definition, fiction writers lie for a living.”

Why our future depends on libraries, reading and daydreaming (2013)
Context: We writers – and especially writers for children, but all writers – have an obligation to our readers: it's the obligation to write true things, especially important when we are creating tales of people who do not exist in places that never were – to understand that truth is not in what happens but what it tells us about who we are. Fiction is the lie that tells the truth, after all.

Source: The Last Werewolf

Source: Children of the Mind

This quote is frequently purposefully misattributed to Lincoln or others long dead before the age of the internet in order to emphasize its point using humour; not all such attributions, or other claims, found on the Internet are as obviously flawed. " "Cite and sound: the pleasures and pitfalls of quoting people", by Tom Calverley, The Guardian (14 October 2014) http://www.theguardian.com/media/mind-your-language/2014/oct/14/mind-your-language-quotations
Variations:
Don't believe everything you read online.
Don't trust everything you see on the Internet.
Everything you read on the Internet is true.
The trouble with quotes on the internet is that you never know whether or not they're genuine.
Misattributed

Source: Opus Posthumous: Poems, Plays, Prose