The Paris attacks in January and November were among the key news events shared on Twitter this year, the social network has said in a roundup of 2015 trends.
The hashtags #PrayforParis and #JeSuisCharlie were among the top 10 news trends cited by Twitter on Monday, referring to the deadly November 13 rampage in the French capital and the killings at the Charlie Hebdo magazine on January 7.
Meanwhile, the #BlackLivesMatter message came to represent a social movement, becoming a rallying cry for social justice after killings by police of blacks in cities including Ferguson, Missouri and Baltimore, Maryland.
But the most-retweeted single message was about the British pop band One Direction. Band member Harry Styles' message of gratitude to fans following news of the departure of Zayn Malik was retweeted 726,000 times, and liked by 744,000.
US President Barack Obama's tweet following the Supreme Court's affirmation of gay marriage, using the #LoveWins hashtag, was another widely-viewed message, retweeted some 448,000 times.
And Saudi King Salman's message of prayer and support to his people shortly after inheriting power was retweeted 368,000 times.
Caitlyn Jenner, the former Olympic athlete who changed gender to become a woman, had her message of introduction retweeted 259,000 times. Jenner set a new record, becoming the fastest person to reach one million Twitter followers - in just over four hours.
The most used news hashtags were #jobs, #Quran and #ISIS, while the top technology themes were #iPad, #SoundCloud and #Android.
The stars most talked about on Twitter were American singer Ariana Grande, Argentine singer Oriana Sabatini and Mexican actress-singer Dulce Maria.
"As always, the world united this year in moments of triumph, activism, support, and fascination, and Twitter is where we gathered for all of it," Twitter's Alexandra Valasek said in a blog post.
"Whether people were making a hashtag into a global movement or expressing wonder over the color of a dress, we all used Twitter this year in awe-inspiring ways."