Facebook's popularity among children fell during 2018, according to the media regulator Ofcom.
It has published its annual report looking at the types of services and devices children in the UK are using.
Its report suggests 72% of 12- to 15-year-olds with a social media account use Facebook, down from 74% in 2017.
But Instagram, which is owned by Facebook, gained popularity. In 2018, 23% named it as their main social network, up from 14% in 2017.
Facebook was named as the "main" social network by 31% of 12- to 15-year-olds, down from 40% in 2017.
WhatsApp, which is also owned by Facebook, and rival Snapchat also saw increased use among 12- to 15-year-olds.
Although most social networks do not allow children under 13 to register for an account, the report found 18% of eight- to 11-year-olds had a profile of their own.
Less than a third of parents who knew their child had a social media profile could correctly state the minimum age limit of the social networks.
Teenagers were also aware of the pressures of social media, with 78% saying they felt there was pressure to "look popular".
Ofcom's annual report is compiled by analysing the media use of about 2,000 three- to 15-year-olds across the UK.
Its latest research suggests 94% still use a television set, although they tend to spend less time using it to watch TV programmes.
Time spent online was broadly the same as in 2017, although average TV viewing fell by eight minutes a day.
For the first time, children were asked whether they used subscription streaming services such as Amazon Prime Video, Netflix and Now TV.
Among 12- to 15-year-olds, 58% watched programmes on subscription services, while 89% of this age group watched YouTube regularly.
Children were also more likely to watch vloggers than in 2017, with 52% of YouTube-viewing 12- to 15-year-olds watching vlogs.
Parents were also asked questions about their children's media habits. Just over half of parents felt the benefits of the internet outweighed the risks.
Ofcom's snapshot of five- to seven-year-olds
- 5% have their own smartphone
- 42% have their own tablet
- 97% watch television programmes on their TV set, for an average of more than 13 hours a week
- 44% watch television programmes on other devices
- 63% play video games, for an average of about seven hours a week
- 82% go online, 67% of those mostly using their tablet
- 44% watch subscription video services such as Netflix, Amazon Prime and Now TV
- 4% have a social media profile
- 70% watch YouTube
Ofcom's snapshot of 12- to 15-year-olds
- 83% have their own smartphone
- 50% have their own tablet
- 90% watch television programmes on their TV set, for an average of more than 13 hours a week
- 62% watch television programmes on other devices
- 76% play video games, for an average of about 14 hours a week
- 53% mostly use a mobile phone to go online
- 58% watch subscription video services such as Netflix, Amazon Prime and Now TV
- 69% have a social media profile
- 89% watch YouTube