According to an article in the New York Times, enjoying using email is a sign you're well on the way to becoming an old fart.
Personally, I'm not convinced. Now while I sent my first email message some time around 1978, I never started using email seriously until 1986 or thereabouts. No one much to email you see.
However I've used it extensively since then, but apparently I'm now an old bastard for doing so. That may well be the case but email has for me always had the advantage of asychronicity - so that being a store and forward solution it works well when you deal routinely with people in other time zones, as well as providing a nice little audit trail.
And while I've use instant messaging across timezones, it doesn't work so well when your fellow IM-er is nine timezones away - you kind of need to have someone who's awake to interact with.
So, I don't think that using email is a sign of incipient senility, what it means is that you have a requirement for asychronous communication, be it with colleagues in different timezones or even just being able to send a message out of hours to Parks and Wildlife about a typo in the rego number on our new National Parks sticker (we bought new sticker for our new car, and they helpfully transferred the balance from our previous vehicle and in the process of the update, well Q is next to W on the keyboard ...).
What the story does show is that the iGeneration typically has a small circle of acquaintences, mostly in the same locale, that they text to about parties, meetups, school and such like. They use text because it's cheap to use, and so naturally make the switch to text like messaging on Facebook.
They need instant response.
An example. If you want to know if someone fancies a beer after work, you're more likely to text them than email them, especially if they're in the next building and you're not sure if they're in this afternoon. On the whole you don't want a reply in three days time - the moment has passed.
Twitter originally looked like it would turn into a service to broadcast social updates. So rather than SMS half a dozen people about you're sudden deep fascination for a middy of VB, you would tweet your followers about your sudden craving. Facebook messaging sans Facebook.
But, interestingly, twitter hasn't turned out like that. While people do use the direct message feature as an SMS replacement (I'm on the train!) it's clear that people are mostly either using it as a curated RSS feed of interesting links, such as my own (@moncur_d) or as a status update service (@UoYITservices as an example) or for live blogging events such as press conferences and presentations.
Twitter has turned into a curated broadcast service. You follow Fred because he has a knack of posting interesting things about papyrology, you don't follow Debbie, even though you're friends with her, as she doesn't post stuff you find interesting, and you while you don't follow qantas you always do a search to check for flight delay notices ...
So, in short, the key take aways are (a) that the communication media used are a reflection of people's lives, and that as people get older they have more and more professional and non social interactions, that require a communications medium that is both asysnchronous, and traceable. Not so much "I'm on the train" and more "I'm on the train and being looking at your project design and ....", and (b) the communications medium used is appropriate to the purpose of the communication.