How to enable Facebook chat heads on iPhone?

I do not have the new option for the pop up chat heads on my iPhone 5,
How can I enable it without any Jailbreak or additional apps?

It must be possible, my colleague who also owns an iPhone had the chat heads as soon as they were updated.
And yes, My Facebook and Facebook messenger apps are up to date.

You have to use latest version of suitable browser for your phone

Never Heard of the Pop Up Chat Heads Ask The Person Who Has it on there Iphone How they Did it

Sadly its not available for every user, I and couple of my friends have the same issue, first i thought its a fault of the app and I tried log in from a different profile and it worked on the same app but not for my profile and rest of the not working profiles as well.
Facebook says that it will be updated to all users soon so best thing is to wait or you can use a different account: D