What about Facebook pages? G+? Tumblr? Pinterest?
If you're looking for agencies who have it down I'd recommend Ogilvy Social W+K Social and RGA. But it really depends on how you want to position your agency, you'll see they all take pretty different approaches.
Before looking at other examples I'd recommend figuring out who exactly you aim to be or who your customers want you to be and go from there. There are hundreds of ways to do social and the only definition of right or wrong that counts comes from your clients.
Dont remember their accounts but look for them! Their blogs are even cooler!
Also check out @SmartInsights they have an awesome following and are UK based. Their blog contains lots of useful tools and templates too.
And whilst not an agency, @mashable is a good example too.