Runescape is an MMORPG, Massively Multiplayer Online Role-Playing Game, which means that communicating and interacting with other players is part of the ethos of the game. Jagex has made this possible in many different ways. Public chats let you meet new people and interact with the population of Gielinor. The clan chat system, while not yet perfect in its design, provides the basic function of allowing people to group themselves into clans and small communities. And while many people would disagree, the Runescape Official Forums do allow players to meet and communicate, albeit not always in the most functional or practical way.
It isn't all limited to talking either. Minigames such as Pest Control, the Great Orb Project and Fishing Trawler encourage players to work as a team towards a common goal. Even competitive minigames like Fist of Guthix, Soul Wars or Stealing Creation bring players together, making them interact in some fashion. Whether that be telling another person on their team to 'rush obby' in Soul Wars or giving a good ol' 'gf noob' to a lesser foe.
One thing that Jagex doesn't accommodate for, and perhaps what is the most effective and absorbing way that Runescape can be played with other players, is through real life interaction. There's something great about sitting opposite a friend in real life and discussing where to go and what to play next. There's no waiting for them to type back to you, you can ramble and rant on as much as you like without needing to stop because you've written more than a line, and you can swear colourfully and loudly with no filters when you get PKed out of a million gold pieces worth of stuff. It's rather interesting that one of the best ways of playing Runescape is a way that Jagex can't manage nor create. This form of playing Runescape is something that is created outside of the game.
It's not just restricted to playing with people in real life either. Third party programs such as Ventrilo or Skype allow voice chats between two or more players. As you can imagine, this is a far more efficient way of communicating with players, particularly in high intensity combat situations such as Clan Wars and PKing. These again are methods of communicating that aren't accommodated by Jagex. Using third party programs is discouraged by Jagex and is shrouded in ambiguity. For example, in the case of SwiftKit, there have been conflicting comments about the 'legalities' surrounding the use of the tool. Jagex attempted to prevent the use of third party programs by incorperating their features into the game, such as clan chats as a replacement for IRC, but Jagex will find it nearly impossible to integrate the tools that Skype or even real life interaction provides the game. For example, how are you going to be able to filter out swearing from a voice chat?
The features that third party voice chat programs provide can't be replicated in Runescape, so what does this mean for the people that do use it? Using such software can be seen as having an advantage in the game, but the same could be said for someone who uses a calculator to figure out how many bows he will need to fletch to reach his next Fletching level, or someone who uses MSN to chat with friends rather than the Runescape PM system. In this respect I don't see these programs as breaking Jagex's rules; for instance, Skype isn't going to woodcut yews for you. Tools like these allow players of the game to interact in a way that is almost like real life, to be able to get that immediate reaction from a friend rather than waiting for them to type. This gives the game a whole new dynamic; one that Jagex can't replicate on their own.