COSMOS is the social network for EVE Online slated to arrive with this winter's Dominion expansion, which CCP Games describes as "EVE away from EVE". Ultimately, it's a way to extend the game experience into new platforms such as web and mobile, along with voice integration -- but this is the long-term plan. In the short term, they've announced a delay with rolling out some of the previously mentioned features when Dominion launches. The initial release will focus on the new in-game mail system being tied into COSMOS and laying the groundwork for other features to come in early 2010.
EVE Online developer CCP Caedmon explained the reasons behind this delay: "We think that this delay for the web application is the right move because we have always had our sights set on the long game for COSMOS. The first release, while crammed with good stuff, was always intended to lay the basis for the ground-breaking innovations to come. Rushing COSMOS out with less polish didn't make sense in this context, we want to impress from day one."
On the topic of day one, CCP Games is aiming to have the Calendar feature ready for the COSMOS launch. Caedmon also provides more details on the various COSMOS features in development. COSMOS will be similar to Facebook in that it'll have a Home section that aggregates various status updates for your friends and corp/alliance mates. Profiles will allow you to view character info as you'd see in-game, and looking at your own profile will give you a look at your character sheet and your training queue. (Our understanding is that the initial COSMOS release will not allow you to switch skills via the web.)
CCP Caedmon's dev blog "Codename 'COSMOS' -- Release Plan and Feature Outline" also details how players will manage their contacts, privacy options in COSMOS, and how 'Watchlists' will allow EVE players to keep an eye on their enemies as well as their friends, much as the in-game Buddies list is typically used.