I've found the RSS feeds can be somewhat flaky, how about setting up a process that would post to twitter with a search tag dependent upon the forum and include the subject? This would just be based upon a new topic being submitted and not based upon individual posts.
Example based upon this topic:
Forum Topic Twitter Feed #SSCSuggest
Other example search tags:
- #SSC2K8Gen - SQL 2008 General
- #SSC2K8TSQL - SQL 2008 T-SQLl
- #SSC2K5Sec - SQL 2005 Security
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