Here are two tools that might help with what you're looking for:
SproutSocial
Hootsuite's uberVU tool
Both of these tools (and a number of others) provide detailed analytics on topics and trends in social media. I'm pretty sure they'll have what you need. If they don't out of the box, I'm sure they'll add it because I can definitely see the potential in what you're asking for
Let me know if either of those are closer to what you need!