Good questions, and it is possible with long term clients and agreements. Its not something that I advise clients to consider for say 3 months or something. That said, sometimes we can kick things off, get them established and help the company in the search for a permanent employee, or help to groom train an existing employee.

I would also add a disclaimer. I think that social media works best when separated from PR.

It can work well together with an overall marketing campaign, but social media often fails when you start mixing the 'official company line' on something with your 'social' conversations.

I think it definitely helps when a company establishes some boundaries between the 'official company line and pitch' and the 'personal conversation and relationship management activities'.

PR is often bad cop to social media's good cop. Merge the two together and you end up with a bad Steve Martin movie pretending to be a Peter Sellers Movie. :)