Sure, following Tweets secretly through your RSS allows a special, stalkerish pleasure, but watching your friend's Internet activity doesn't have to be all about surreptitiousness. When a pal shares a link to her Flickr account or I run across a photographer whose work I find inspiring I stick it in my RSS reader so I can keep up with their activity.
Flickr offers an RSS feed for photostream pages, which you select and add to your reader. You'll find it in the toolbar of your browser if you use Firefox (see previous link for other browser tips). Flickr also offers a convenient "bookmark on delicious" link at the bottom of each photostream page, so you can save a friend's stream in your bookmarks and have access to it from any computer.