MSN

And if you look above ladies and gents, you'll see old foggies pretending to be "with it".

MSN Messenger is an Instant Messaging program. It easy of use, as well as speed, cost and convenience means it is replacing email, text, and telephone calls (among certain audiences).
It is a good example of social networking, a phenomenon that occurs when new media technologies are introduced into audience groups.

MSN Messenger was a freeware instant messaging client that was developed and distributed by Microsoft between 1999 and 2005 for computers running the Microsoft Windows operating system, and aimed towards home users. Amongst its users it is often referred to as "MSN”. The business-oriented client for Windows, which also uses Microsoft's .NET Messenger Service, is called Windows Messenger. It is possible for virtually any computer with an Internet connection, Windows XP and Internet Explorer to connect to the Messenger Service by using MSN Web Messenger, or one of the unofficial equivalents (which have advantages such as compatibility with browsers other than IE, but do not have an interface that mimics the one of the MSN Messenger application). MSN Messenger was renamed Windows Live Messenger in February 2006 as part of Microsoft's Windows Live series of online services and software.

Accessibility and ease of use means that the market is made up of mostly teenagers and below, since they have the most disposable time for interacting with friends and relations. The “instant” message appeals to this young generations needs of more flexible, convenient, faster technologies.

Also, the key reason it has become the most used communication method in the UK among teens, is that its free, paid for by advertising.