In one rant, JWZ pretty well maps out one of the next big areas in social software: calendars!
From Groupware Bad.:
I said, instead of trying to build some all-singing all-dancing "collaboration server" where you're going to throw in all kinds of ridiculous line items like bulletin boards and task tracking and other sh*t, let's suppose you narrow your focus to just calendars.
The first thing you want to do is make it trivially easy for someone to publish their calendar, allowing other people to check their schedule (and, for example, know when our target user has classes, when he's planning on studying at a cafe, what nights he's thinking of going to a movie, and what concerts he intends on seeing). Right now people can do that by publishing .ics files, but it's not trivial to do so, and it's work on the part of other people to look at them. If it's not HTML hanging off our friend's home page that can be viewed in any browser on a public terminal in a library, the bar to entry is too high and it's useless.
Then the next thing you want is an invitation manager like Evite but that doesn't suck. Evite sucks because they're spammers, and because it's more important to them to put advertising in front of your eyeballs than to be useful, so the mail they send out doesn't actually include any information, in a lame-assed attempt to drive hits to their web site. So what you want next is a free replacement for Evite -- but more to the point, one that doesn't require any kind of server running anywhere.
And if it doesn't work with webmail, you've lost before you've even begun, so don't do something dumb like requiring a plugin. The trick you want to accomplish is that when one person is using your software, it suddenly provides value to that person and their entire circle of friends, without the friends having had to do anything at all. Then, later, you pull the friends into the fold: if one of them starts using the software, they become their own hub, and get the benefit they have already witnessed from a distance.