The idea is that someone nominates you to name three positive things a day, and these could be nice things that have happened to you or things you are simply grateful for (like that last After Eight you didn't know was there).
But while we have a very clear steer on negativity, positivity is a bit harder to define. In fact, if you think it's being cheery all the time like a dancing elf on MDMA, that definitely isn't the definition of it.
Dr. Kirsten Harrell, a psychologist and life coach, who founded International Positive Thinking Day, says she discovered the effectiveness of positive thinking while dealing with chronic pain.
Dr Sheri Jacobson, clinical director at Harley Therapy says: "Positivity can elevate our moods and broaden our perspectives - instead of just seeing what is going wrong, we see what is going right. It can help fuel our accomplishments and raise our self-esteem."
However, adds Dr Jacobson, positivity shouldn't be used to mask your feelings. Which brings us to that falsely cheery persona put on by some people going through a difficult time. Sean Dagan Wood, editor of Positive News, agrees.
"it’s important to be clear that positivity is not about burying or glossing over difficulties or suffering. And positivity isn’t something that’s always appropriate - sometimes we have a need to just be with whatever we’re experiencing without responding to it in any particular way. But I find that being able to respond positively to situations - which in reality is sometimes really tough - can change my whole experience.
"This starts with self awareness and the belief that how I view a situation and the intention I create and act on in response, will directly influence what happens next (though the outcome is unknown). This is the power that we all have to affect change in our own lives and the world around us."
Positivity doesn't come easily however, which is why Facebook challenges are great to make you proactive.
Dr Jacobson adds: "Our minds naturally tend to troubleshoot - going over things that have already happened that didn't go well, or worry about the future. Many people will not focus on the positive (and may not even notice it), without training."
"If we don't make the effort to notice positive things they can get lost in the general noise of life," says Vidyamala Burch.
Burch, founder of BreathWorks, shares a similar story to Dr Harrell, in that she too suffered from persistent, chronic pain and used mindfulness to manage it.
"At the end of the day we may have some vague awareness of positive experiences having been part of the day but they will be indistinct and just part of the general soup of experience.
"So, positivity challenges are a very good way to make sure we actually notice positive experiences when they happen. And that noticing will mean the positive experiences also have more of an impact upon us - they will actually feel more positive!"
If the thought of using Facebook and announcing your daily positivity leaves you cold, Wood suggests keeping a gratitude diary.
"Practicing that then makes it easier to cultivate that state of mind and find a path out of excessively negative thinking when you get stuck in it. It builds resilience.
"I think gratitude helps us see that despite the suffering that life can entail, all of life is a gift. So gratitude can put things in perspective, help us to realise what really matters and help us experience being part of a wider whole. By transcending ourselves we can sense the intricate connections between everyone and everything, and for me that’s a route to feeling alive."
One thing we shouldn't do, however, is to confuse positivity with happiness. They may be interlinked, but are not the same thing.
Danny Penman, co-author along with Burch of Mindfulness for Health: A Practical Guide to Relieving Pain, Reducing Stress and Restoring Wellbeing says: "If you want an immediate boost to your happiness levels the quickest way of doing so is to simply focus your attention on the world around you.
"For example, if you’re walking down the street pay attention to all of the sights, sounds and smells washing over you. Notice the trees and the flowers or perhaps all of the multi-coloured shop fronts. How do your feet feel as they touch the ground? Can you feel the wind, sun or rain on your face? Simply paying attention like this will help you engage with the world again. It is this engagement that is the source of happiness and contentment in all of our lives."
However, while positivity may not lead to happiness, Burch says it can work the other way round. "They are mutually reinforcing - happiness leads to more positive action which leads to more happiness. When positivity becomes a habit then happiness becomes a habit."