- vBulletin provides an RSS feed but it's limited to new threads. It doesn't include new responses and there's no way to filter it to only certain forums. It also has to be turned on by the forum maintainer first.
- Blogger has Atom feeds which include links to feeds for the comments of individual entries, but most of the readers I see for Linux don't support them.
- Both MySpace and Facebook have several feed reader applications (third-party plugins) but neither provide feeds. Anything that would normally be posted in the 'friend updates' would be very useful for exporting.
- LiveJournal has Atom feeds of individual journals but they don't include feeds for comments on individual entries. Getting a feed of protected entries (through a authenticated gateway) takes some extra (understandable) work. They also don't provide a feed of friend list or friends of friends list. Someone implemented a style override that emulates an RSS feed but it doesn't work for the friendsfriends page. The emulation also can't provide information about comments each entry.
If I can't keep all my updates in one place then chances are I won't be hanging around a site for very long. I don't like having to load a dozen websites (tabs) and log into each one individually just to find out that only three of them have updated since the last time I visited.
Many people are moving toward web-based services such as Google Reader or even centralizing their feeds on Facebook, MySpace, or LiveJournal (which I was doing). I don't trust web-based services and would rather use a client to aggregate various feeds for local consumption. I especially don't care for Firefox's 'Live Bookmarks' feature that requires to you click on each one then manually compare (remember) the state the last time you loaded it to see if there are any new entries.