Mark Rogers of Market Sentinel made the case for evaluation. You can measure themes in conversation, brand mentions, authority and approval.
It comes down to 'where are you on Google'? Organic search is key. Contextual links are the answer. This makes conversational networks important.
Social media marketing and online PR are the same thing.
Measuring influence is difficult: you can do it by tracking conversations and citations. Bloggers aren't normally very important - except when issues arise. Bloggers are gateways for conversation.