Ignoring social aspects for a moment, one of the effects has been to reduce the level of boredom I ever feel (obviously combined with working full-time of course). I seem to remember the summer holidays as a teenager being spent doing the most inane of tasks simply to kill time. Because I would run out of movies to watch, or games to play I eventually ended up doing things either vaguely useful or utterly pointless.
Now this
never happens. I'll never fully play every game I have on our network, nor listen to all the MP3s I've got currently or watch all the movies - let alone the umpteen gig of new content we seem to obtain every day. Even beyond this, there's always a web page to read or a flash game to play. Discounting time at work, I never feel bored.
The consequences of this are numerous but in particular :
1. Positively, it means you don't have to put up with crap anymore. Terrible SNES games would have been played right through to the end because you had spent £40 on them and shitty TV programs would be watched endlessly because there was nothing better available. Now the standard something has to reach (in terms of praise from others) before I'll watch/listen/whatever is ridiculous high.
2. Negatively, this combined with other factors has utterly destroyed my concentration span. I require stimuli all the time, and it's not uncommon for me to listen to music on my way to work while reading the paper and periodically checking my phone.
More pertinently, there are movies sitting on our server which are undoubtedly classics but I just never seem to get round to watching. Why? Because I am never bored enough.
When I was 12 or so I happily sat down and started reading Lord of the Rings for the first time and yeah, the first 60-70 pages are pretty dull but so what? It's either this or watch the Brittas Empire. Again. If I was approaching the book for the first time now, I would probably give up before Bilbo's birthday party has finished.
In short, boredom probably serves an important psychological function. Now if I'm on a long train journey with nothing to read or listen to, I actually feel very uncomfortable - I've never been a smoker but from what I can tell it's like craving a cigarette. I've heard other people here say this, but often if I'm waiting for a bus I'll walk to the next stop rather than wait (even if this means I frequently miss the bus between stops). So perhaps the internet has made me some sort of content junkie, I'm not sure.
Socially speaking I've found the impact different from I originally expected. I think people (or maybe just me) expected "internet friends" to be the main consequence of electronic communication. The early days of .Net magazine (when they used to print how many people globally had internet access - in the low millions I think) they would talk of having friends around the world. I think I sort of expected to have penpals around the world. And to an extent, that's happened - my frame of reference now includes fellows I know from Brazil to Stockton (on Tees & California).
But the main impact socially has been on the people I already know/knew and on new "real-life" acquaintences I make. If I meet somebody when out clubbing I might strike up an email correspondence with them, or "bump into them" on a shared forum. And this coincides with a general trend. Internet socialising has gone away from "Hey you can contact people anywhere in the world who might be totally different to you" to "Hey you can find people in your area who are interested in the things you are". Partially this is because there's a lot more people online - it would have been hard to find someone into very similar things to you who also lived near you when there was only a few million people who used internet services - now you can. Of course, I speak from only personal experience here - others might still find that 99% of their internet communication is with people in a different country - it'll depend on what sorts of things you're into.
Anyway, the internet has made me a lot more social. I'm not one for phoning people up socially just to chat, which means ocassionally I let social links atrophy if I don't regularly see someone. The internet has meant it's much easier to renew these links (and develop new ones) because there's low cost methods of keeping in touch (cost in a psychological sense that is).
This leads to strange consequences though - I went out with a girl for a year and a half, we saw each other 2-3 times a week during that period, but I don't recall phoning her (beyond a "I'm running a little late" sort of call) more than once or twice during the entire relationship. Does that matter? Probably not.
edit : As you may have noticed, the internet also helped me become an insufferable bore.