Huh? What's a secondary activity? Well . . . where's my chalk?
To start with, a primary activity is the one you're, well, primarily engaged in. If you're blowing the snow out of your driveway, that'd be your primary activity. A secondary activity for that would be having to mix gas and fill the tank.
A secondary activity is one that either sits between you and the primary activity or serves as an interruption to it. This can change with context and who you are, but it doesn't change the fact that secondary tasks are irritating: usually highly tied to how important and urgent the associated primary task is.
I brought this up on Sunday at breakfast with regard to bookmarking on del.icio.us. For *me*, boomarking is ALWAYS a secondary activity. When I bookmark a site, I'm always "doing" something else: researching a problem, reading an article, looking for a new tool, etc. I have never and will likely never sit down at the computer and think, "I'm going to bookmark some sites for the next hour".
The normative way of using del.icio.us is to click the button, fill in the description, choose some of the suggested tags, add a few of your own tags and submit the link. That's a LOT of steps for something that's a secondary activity.
And, it's usually an interruptive secondary activity. I often decide I'm going to want to save an article when I'm half-way through reading it. If I save it when I have that thought, I get dumped back at the top of the page after bookmarking. However, if I don't bookmark it immediately, I'm highly likely to not follow through.
Now, before people start suggesting Firefox extensions, better bookmarklets, etc., I know about those (and am pretty happy with my current setup). However, that doesn't change the fact that the default way of using that service doesn't match the situation most users are in when they'd need it.
If your primary product is used mostly as a secondary activity for people, you had better be making it as smooth as possible or you're losing out on lots of users. The pattern you'll likely see in this case is people who sign up for your product or service, use it heavily for 2-3 days and then disappear.
That pattern happens as people realize that the effort is just too much for a secondary activity. They clearly see benefit (or there wouldn't be a burst of activity originally), but they don't see *enough* benefit to compensate for the effort.
That's also one of the reasons I'm not a fan of forums and prefer email lists as well as RSS over visiting a roster of sites regularly. See, despite wanting to keep up to date on these topics, the act of sitting down to deliberately go through a list of sites and "catch up" just never rises to the level of a primary task (it's not alone in that).
However, when those things are delivered as an effortless secondary task, I can keep up and don't get irritated. All of that new information is delivered right to my email client, where I *do* spend time doing primary activities. It's automatically updated, sorted and filtered. Those automated tasks reduce my effort below the threshold where I'd abandon other secondary tasks.