One of the big issues for blogs these days is that most authors also generate a lot of online activity away from their weblog (this wasn't true in the pre-Facebook/Flickr/YouTube days). A great option I've found for Moveable Type users is
Action Streams, which I'm using to power the "Elsewhere" section at the right side of my home page. The items aren't posted into the weblog, however, as per my old "link for date" posts from de.licio.us. At first I considered that a shortcoming (I wouldn't mind Twitter comments showing up in my blog) but for the moment I'm in favor of it, as I've realized that the combined activity on other sites would overwhelm the edited content in the weblog.
One problem I'm starting to encounter is that when you use meta-social sites like
FriendFeed, which aggregate activity from the social sites, you can start seeing some circularities in your feeds... my FriendFeed feed follows my
twitter and Facebook profiiles, but it's also able to post events into my Facebook profile via a plugin, and... uh... problems start to arise.