While messages of love and gratitude shared by One Direction topped Twitter's 10 most retweeted charts this year, a few serious tweets also made the list - although few are likely to be remembered for heralding pivotal moments in history.

Of 2015's 10 most shared tweets, five were from personalities other than 1D members. One of those came from US President Barack Obama. The POTUS' message celebrating the legalization of same-sex marriage across the US was the first non-1D tweet - coming in at fourth most tweeted.

At number six was a message of prayer and support for his people from Saudi Arabia's King Salman after his crowning.

"Dear courteous people, you deserve more. No matter how much I do, I will not be able to give you your dues, I ask Allah to help me serve the faith and the homeland, and do not forget me in your prayers," wrote the king in his native Arabic.

In September, hip-hop personality Kanye West called for people to do everything they can - some fairly straightforward advice. His tweet came in eighth place.

Just days before his death in February, pop culture icon and actor Leonard Nimoy tweeted this emotional message to his followers, the ninth most retweeted tweet of 2015.

Caitlyn Jenner introducing herself to the world after transitioning from one-time Olympic decathlon champion Bruce Jenner rounded out the top 10 most retweeted content of 2015.

For the record, the most popular tweet of 2015 came from One Direction's Harry Styles in a message he wrote to fans after Zayn Malik departed the band in March. With more than 720,000 retweets and 744,000 likes, it's clear the 1D boys have some serious pulling power when it comes to social media.

#JeSuisCharlie, #BlackLivesMatter and #LoveWins

Twitter also released a list of some of the most popular topics of the year, denoted by the hashtags which resonated with more than 300 million users of its service.

#JeSuisCharlie, #JeSuisParis and #PrayForParis were the most tweeted hashtags of the year, marking the times social media came together to show solidarity in the wake of terrorist attacks in the French capital.

#BlackLivesMatter also made the list after police shootings of African-Americans across several US cities. The hashtag was tweeted 9 million times in 2015. It now serves as a unifying message for communities discussing the events of #Ferguson, #Charleston and the #BaltimoreProtests.

The hashtag #LoveWins came in fifth after the US Supreme Court ruled on gay marriage in June.

#RefugeesWelcome became the top trending hashtag for people from the Middle East seeking refuge in Europe. Subscribers across the Twittersphere used the hashtag to call on European countries to grant refugees entrance.

Global conversation

When the world learned of 14-year-old Ahmed Mohammed's arrest at his school in Texas in the US, the Twittersphere exploded with the hashtag #IStandWithAhmed. The hashtag sparked a global conversation about racial profiling and discrimination.

Other popular events on Twitter this year included Pluto's flyby, the Women's Soccer World Cup, along with elections in the UK, Argentina, Canada and India.

'Face with tears of joy'

And, let's not forget this year's top emoji's - one was so popular it was even named the Oxford Dictionaries' word of the year.

Despite Twitter's large audience, Facebook's social network is five times larger. A comprehensive list of what users were talking about on Facebook in 2015 is expected to be released before the end of the year.