This is nearly impossible to figure out. As the values will always be changing. Players leave and players come. It's mostly all about the randomness of how people come into the game.
No, it's not. It's possible and important to understand if you want to have a commercially successful game. It's hard to predict individuals, but you can get very good numbers on groups.
For example, on NearbyGamers.com, a very low percentage (<10%) of visitors who find the site via Google sign up, and send few messages (<5) to other players. Visitors who follow a link from any gaming forum - are more than twice as likely to sign up and then are more engaged, they send more messages. So I'm spending almost no time thinking about SEO and a lot of time thinking about how to get gaming forums talking about the site and linking to it.
The longer a player uses a game, the more chances they'll spend money or spur their competition to spend money. It's important to know how long the average person plays to find out which traffic sources are worth more attention - maybe players who find you through Google will play months longer than players from a voting site. So you know which players you want to spend more time getting, and you also know when you should contact your players to try to reinforce their involvement with the game.
I strongly recommend Dave McClure's "Startup Metrics for Pirates: AARRR" presentations, he explains what numbers are worth thinking about and, high-level, how to start using them well. You can read the
slides watch a recent
video.