Image source: Facebook.

Two and a half years ago, everybody freaked out when Facebook's (META -0.43%) then CFO, David Ebersman, said the company was seeing a decline in daily usage among younger teens. To be sure, 2013 marks the beginning of the decline of Facebook as teen's "most important" social network. But that doesn't mean teens and young adults are no longer visiting Facebook.

A recent report from comScore shows that 18- to 34-year-olds in the United States use Facebook more than any other social network. That's not really a surprise, of course. Facebook counts 1.6 billion monthly active users around the world, which is 4 times bigger than its closest competitor, Instagram. More interesting is the fact that according to the comScoare data, millennials spend 2.5 times more time on Facebook than on the next-closest social network, Snapchat.

More than 30 minutes per day
The average Facebook user spends over 1,000 minutes a month -- about 33 minutes per day -- on the social network. The next closest competitors are Snapchat and Instagram, which both hover between 300 minutes and 400 minutes per month. The average Twitter (TWTR) user spends less than 200 minutes per month on the platform, according to comScore, and the average LinkedIn (LNKD.DL) user spends almost no time on the network.

What makes that number more impressive is that nearly every millennial with access to the Internet uses Facebook. So, when Sheryl Sandberg says, "We have a Super Bowl on mobile every day," she's hardly exaggerating. With the average millennial spending one sitcom's worth of time on the platform each day, few other platforms, even television networks, can claim the same reach and engagement on a daily basis.

Sure, Facebook might not be cool anymore, but it's become a utility of the Internet like email. You can live life without it, but it's inconvenient. And while most teens say Facebook isn't their most important social network anymore (Instagram takes that title), it still grabs a lot of their attention on a daily basis.

Twitter can't compete
Just as striking as the data on Facebook is the amount of time Twitter and Vine users spend on their respective social networks. Twitter users spend an average of only six minutes per day browsing timelines. The numbers are worse for Vine, Twitter's 6-second video app. Just 20% of millennials say they use the app, and those that do barely spend any time on it. Granted, you can watch most Vines on Twitter.

Engagement has been a problem for Twitter for some time. The company added Moments last year to highlight its best content, draw in new users, and increase engagement, but there's no indication that it's working. It also recently changed its timeline to show the most interesting tweets first.

comScore doesn't have any data on Periscope -- Twitter's live-streaming app -- but indications are that it's catching on. The company just reported its users stream nearly 1 million hours of broadcasts every day, up nearly 3 times from August.

Does LinkedIn need engagement?
The only social network users spending less time on average than Vine users are LinkedIn users. But LinkedIn's business is very different than Facebook or Twitter's, which rely on users' eyeballs gobbling up advertisements. LinkedIn generates the vast majority of its revenue from Talent Solutions and Premium Subscriptions.

But LinkedIn is actively focused on growing its Sponsored Updates business, the biggest part of its Marketing Solutions segment. Moreover, it shuttered its ad network, which allows it to leverage user data across devices, apps, and websites, which means it's more reliant on direct engagement to grow its ad business than before.

While LinkedIn isn't as affected by poor engagement as Twitter, investors would still rather see users visiting the site more often or LinkedIn leveraging its unique user data to drive ad revenue outside of its core product.