Topsy, a search engine for Twitter, claims to have the biggest index of tweets, and is building out its infrastructure to handle real-time updates. (So is Google.) Topsy’s version of page rank appears to be a relatively simple notion of influence, but its search results of Twitter content seem more relevant than Twitter’s own, or Google’s. But I’m skeptical there is more value in conversations than in documents, especially over time. Don’t get me wrong – there’s a critical need to filter real-time info streams, and there’s utility to adding tweets to a search engine’s relevancy algorithm. But how valuable is the tweet archive?