A WebmasterWorld forum thread asks Do article links expire? He asks this because he noticed a drop in backlinks recently. After skipping over the first few posts, I noticed WebmasterWorld moderator sugarrae, respond. She said that she noticed the same "thing this week." She made a distinction between "out of the box" article directory sites and sites that have an "editorial process" when accepting articles. The links from the sites that have the editorial process are still worth something, she explains. Whereas the other "out of the box" article directory sites are not.
Article sites weren't a mass link building tool a year ago. And much like everything else, things being abused usually inspire changes in search engine algorithms.
Forum discussion at WebmasterWorld.