Facebook's collection of data has come under increased scrutiny after information on millions of users was leaked by a third party.

The hashtag #DeleteFacebook has trended online and many users have chosen to download copies of their Facebook data to better understand what information the social network has collected about them.

These files, which Facebook enables users to download freely, confirm the company has a wide range of information available on people, beyond the obvious content that is visible on someone's profile.

Some phone call and SMS message history

In going through their own files, several users have reported alarm at Facebook's data-gathering because their files were found to contain phone call and SMS message history logs.

The data included time and date information of calls and messages sent to friends and family.

However, the social network has already responded to reports on the issue, asserting that this collection is not done without first gaining user permission.

Facebook said the measure is an opt-in feature for the Messenger for Android app and the streamlined version of the site - Facebook Lite - and was designed to help better connect users.

"You may have seen some recent reports that Facebook has been logging people's call and SMS (text) history without their permission. This is not the case," the company said.

"People have to expressly agree to use this feature. If, at any time, they no longer wish to use this feature they can turn it off in settings."

"We never sell this data, and this feature does not collect the content of your text messages or calls."

Wider contact information

As well as the list of friends a user has on their Facebook account, the names and contact information - such as phone numbers and email addresses - of anyone saved to a device on which you use Facebook also appear as part of the data.

Again though this is an optional feature, Facebook asks users first if it can access their contacts list as a way of finding and suggesting connections on the social network.

The process is also common across many social media and other apps, usually occurring shortly after first joining a service.

But it does mean some people without a Facebook account could see their phone number become part of the platform's data archive, should any of their friends or family be on the site.

Deleted and declined friend requests

Facebook's data on users includes not just their current friends list and the date that online friendship began, but also a list of friend requests a user has declined and those friends they have removed from their profile.

The data also features lists of friend requests sent by the user that were declined by others, offering a detailed picture on interactions with others on the site.