Sounds interesting. This seems to be a more natural way for conversations to develop. It mirrors real-world conversations where random users can just join and leave.
The analogy to hashtags makes sense. However since hashtag topics can last indefinitely, they can get stale and lose relevance. Allowing tags/conversations to expire after a period of time keeps the conversation topical.
The concept is also scalable, since you can either have a small conversation among friends, or a regional conversation about relevant issues in the news.