Hi Michelle,
Yes, we do use HootSuite. I too remember Marketo saying something about integrating with HootSuite. According to this: http://launchpoint.marketo.com/hootsuite/663-hootsuite Marketo integration is coming soon.
The manual part is an option. The only way I can think of doing it manually is, collecting retweets, figuring out their real names (if possible), finding them in SFDC, and adding to their score. Can you think of a different way?
The Twitter handles need to be mapped to a lead, either by email or name, in SFDC (or Marketo) to count.