So what’s changed? My theory: mostly the internet, especially the rise of social media. I bet there's been research about this, and I haven't looked into any of it. Totally unsourced speculation ahead!
The internet makes us aware of more local events, so we either make more plans or feel like we’re missing out on all this cool stuff we could be doing. This explanation sprang instantly to mind, but on reflection I think it’s probably a fairly small part of the effect -- if I picked up a local paper or took notes about the flyers posted on the library bulletin boards I’d have nearly as much information about local events, and those methods of communication have been around for yonks, so I don’t think slightly increased ease of getting information about public events can explain much of the difference.
Private events, on the other hand, have changed a great deal. E-mail makes it very easy to invite a huge number of people to an event -- and with less lead time. With no need to spend time laboriously addressing invitations or making phone calls, many more people can be invited to an event. And, partly because people are so busy, I find myself inviting a lot more people than I expect to turn up, so that the fraction who do attend make up a large enough party.
And that’s why we feel so busy -- we’re trying to maintain a very large number of social connections, because the technology rather suddenly means we can.
Increasing mobility leads to a fracture in types of closeness. If people in general stay put and lack long-distance communication, then your geographically close neighbors are your easy-to-contact people are probably your coworkers or family are your emotionally close bonds because the pool of available people is just smaller. But these days, your geographically close neighbors aren’t any easier to contact than your family in other states, your emotionally close friends may live in a different country, which people are easiest to contact has more to do with whether they like the same forms of communication you do (texting? email? same social networking website? same IM software?) than with what other forms of closeness you share. The pool of people who are available to connect with has become enormous.
Mobility’s been increasing for several generations now, which weakened the traditional family ties as families spread out and were no longer also neighbors. That led to the creation of other sorts of groups based around the new locations -- fraternal orders, hobby clubs, things like that -- and a general transfer of closeness to friends and coworkers rather than extended family members. Then, once communication caught up with mobility, we started seeing a different kind of shift in social circles. The ease of keeping in touch with those family members (or old friends, schoolmates, etc.) has greatly increased, so that those ties are better maintained. Social media requires action to drop people rather than action to keep up with them, so people don’t so much change social circles as gradually accumulate a loose cloud of tenuously connected ex-schoolmates, ex-coworkers, people who used to live in the same town but have moved, and so on and so forth. Easy communication encourages keeping in loose touch with many people. This means the social graph is spread thin, maintaining many relatively light connections rather than focusing on a few connections at a time and dropping older ones entirely.
Once I got this worked out in my head, I stopped feeling over-busy almost immediately. Just realizing that I was trying to maintain more social connections than I would have been able to have 20 years ago made me feel much better about it without even changing my behavior. I thought for a little while Hm, maybe I should deliberately drop some of the more tenuous connections, pick a group to focus on more, try to return to an older style of engagement and then I realized that I didn’t in fact want to do that -- I like my social connections, the ones that are close along the various possible axes of closeness and the ones that are more distant, and I’d rather have those distant relationships than not have them. But just realizing that they are distant relationships, for whatever set of reasons, lets me relax. I don’t have to make them all close relationships -- I can’t make them all geographically, emotionally, ease-of-contact, etceterally close relationships -- and I don’t need to make them close relationships to keep them. (There’s also something of a coordination problem: the whole group needs to choose to focus reciprocally, or the ones who are trying to focus just drop some connections without being able to strengthen the remaining ones, because the people they’re trying to focus on are still spread out and busy.)
The other possibility is that this has nothing to do with the internet at all and it’s all a result of living in a much larger metropolitan area than I grew up in. That’ll increase your pool of available people, too, for sure! But either way: Once I realize that I’m busy because I know a lot of people and want to spend time with them, I suddenly feel much calmer about the whole thing. It's funny how having an explanation for why I'm stressed about something can just make the stress go away.