Story highlights Facebook now allows teens to post photos and updates to the general public

The company wants to compete against younger social networks and appeal to advertisers

Stricter sharing will be the default, warnings will pop up before post is made public

Facebook is relaxing its rules for teenagers. The 13- to 17-year-old set now has the option to share photos, updates and comments with the general public on Facebook. That means strangers, and companies collecting data for advertisers and marketing companies, will be able to see select posts. Teenagers will also be able to turn on the Follow feature for their profiles, which would allow anyone they're not friends with to see their public posts in the main news feed.

The changes will take effect immediately, the company announced in a blog post . The new setting might help Facebook compete against other social networks that skew younger, and having public data on teens will also help the company appeal to advertisers.

The social network is trying to balance the less strict settings with two other privacy protections. When new underage users sign up for a Facebook account, their posts will be shown to a more limited audience by default -- only to friends instead of friends of friends. If a teen decides to change the setting to Public, she or he will see a pair of pop-up warnings explaining what "public" means. One warns they could end up "getting friend requests and messages from people they don't know personally." Default settings for existing teens with profiles won't change or affect past posts.

Facebook has been around for more than nine years and stopped being a hip place for kids long ago when it was invaded by parents, grandparents and advertisers. It has 1.2 billion users. Even as it expands to all ages, the company has to work to hang on to the coveted teen demographic.

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In a recent Pew study, teens reported "waning enthusiasm" for the social network, citing the presence of adults and drama. The site has become too important to typical teen life to abandon, so 94% of teens on social media have a Facebook account, and the average teen user has 300 friends.

Other social networks such as Twitter, Tumblr and Last.fm don't prevent teens from posting publicly. However, if someone under 18 wanted to bypass the setting on Facebook before today, they could easily lie about their age when signing up for an account. Children under 13 are not officially allowed to sign up for a Facebook account, though they can skirt the rules in the same way. When someone underage does sign up for an account, Facebook assumes they have the permission of at least one guardian but does not verify it in any way.