The social network, along with sites such as Yahoo and Tumblr, is adopting the colour to stand against bullying and show support for lesbian, gay, bisexual and transgender (LGBT) youth.
The profile pictures on Facebook's Stop Bullying: Speak Up, Diversity and Safety pages are now purple. Other Facebook users can turn their pictures the colour by adding a purple twibbon to them.
Facebook has backed Spirit Day since 2010. Gay & Lesbian Alliance Against Defamation (GLAAD) co-ordinates the day as a way to "acknowledge the pain that all of us in the LGBT community and our allies have felt with the loss of so many young lives due to bullying".
Alongside turning the pages purple, Facebook is also encouraging its employees around the world to wear the colour today.
All Spirit Day participants are urged to upload photos of themselves wearing purple to Instagram and Twitter, using the hashtag #spiritday.
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Various celebrities have already expressed their support for the day, including Ian McKellen, Sean Maher, Rita Moreno and Fran Drescher.
William Shatner tweeted: "I'm purple in the face in speaking out against bullying especially on 10/19. glaad.org/spiritday MBB."
In a message posted yesterday (October 18), Stephen Fry wrote: "I'm wearing purple tomorrow to show some support to LGBT kids. Join me for #SpiritDay 10/19! glaad.org/spiritday."
Meanwhile, teachers have spoken out against an emerging trend of pupils posting their "dirtiest secrets" on Facebook.
It now has 600 fans and features a range of crude, degrading and explicit messages about pupils at the school in South Yorkshire.
Teachers are concerned that such pages expose the students to potential abuse and also bring schools into disrepute.
Geoff Baddock, senior deputy head at Penistone Grammar School, told The Daily Telegraph: "Our local authority is working with Facebook to tackle this because it is appalling. We're taking action to warn our students that it is inappropriate and they are putting themselves at risk.
"We would always take appropriate action against pupils who are putting the school into disrepute and support students who are at risk. One of the aspects of social media is youngsters post things without realising the consequences of it."