Twitter provides essentially a linear, one-dimensional stream of updates. You follow the people you're interested in the updates from, and you don't follow the ones you're not (though people can still essentially contact you by addressing a tweet to you). Back-and-forth conversations sometimes take place, despite the primary function of twitter being advertising your present status/doings. Nevertheless, there's a general limit to how many updates you can receive before it just becomes noise.
Astonishingly, people seem to keep watching despite my twit-stream being largely concerned with errands, sleeping, waking up, drinking coffee and writing.
Plurk (like Pownce) is two-dimensional. Every plurk is a potential conversation-starter, and each plurk notification can become its own thread of updates/responses. It's obvious that there are only so many conversations you can pay attention to at once, and they are moving further and further from being in front of you as time passes.
With the potential for each item to have an associated commentary and conversations from others, it isn't actually clear what purpose a plurk serves. Twitter's "what are you doing now?" is as easy to grasp as the @doing command in early generation virtual worlds(Although @doing is a term which you'll have a lot of trouble looking up, as most search engines won't let you find things prefixed with an @ and strip it off).
Plurk lacks the same precise focus of purpose of Twitter. Yes, that's certainly not all that people actually do with Twitter, but it's nice to have a precise purpose to fall back on :)
Depending on how chatty the Plurk users you are following actually are, it seems you can follow far fewer of them effectively than you can follow twitter users before the number of responses simply becomes unmanageable. Depending on the person (and the people they're following) the number could be as low as three or four people.
As the numbers grow, you become increasingly less a participant in conversations, and more of a spectator. Beyond that, you probably aren't going to be able to keep up, and you'll burn out, much as a few people I know have done on Pownce.
So, overall, while you have to be somewhat discriminating to keep up with Twitter, you have to keep even tighter reigns on who you're following on Plurk just to keep up. If I'm not following you on Plurk, or I stop doing so, it's basically because I'm trying to manage the number of Plurk updates per hour that are coming in. There may already be more than I can handle alongside other tasks.