Last I heard there were close to 125,000 active accounts, usually with about 30,000 players logged at once during weekdays, about 35,000 during weekends
To really experience all EVE has to offer, i say you need to spend at least 2 years, because of the time it takes to train the advance skills, about 10 months to really learn all the advanced and finer details (and poorly documented, sadly) of the features of the game.
If you want to get involved in politics, i say about 1 year before you can start taking an active part in the politics of the game, 2 or more years to really start influencing the monster, so to speak.
Social Skills are pretty important in this game as well. I'm sure some may argue you can solo but IMHO no-you-can't. You really need to be a part of a corp and be fairly active in a social way. Kind of the way WoW you need to be on 4-5 days a week and be active in your guild.
Yep, EVE is team based game, the real "meat" of the game lies in joining a group of players. Most activities revolves around teams. So you will definately have the most fun for the longest time if you join a good group of players.
Thats not saying you wont have fun soloing, but after 6 months or less, soloing becomes rather boring. Where as if you join a group players, you can do many diferents things which will extend the life of the game greatly for you.
One last thing, notice i said "GOOD group of players", eve if full of people who will scam you, betray you, steal, kill you, extort (all in game of course) simply because they can. You will have to have a lot of patience and learn to adapt. To have fun in EVE you need to ADAPT.
EVE is hard and ruthless, but fun

Personally i played for about 3 straight years, great game, but for me, after three years, the rough egdes of the game really caught up to me, and having already done so much, things started getting a little dull, not to mention that the politics and administrative things i did took much time and werent much fun, so i decided to take a break and play other games, yet still havent decided to go back, first im gonna give Age of Conan a chance