As I mentioned in my webinar last month, Search Engine Optimization is useless if your site is not interesting to real people. Site visibility has some additional thoughts on situations in which SEO and usability can be at odds with each other. My general rule is: Focus on the end user above all. If an SEO tactic takes away from the user’s experience, it is not worth implementing.
SEO Gadget explains an advanced wordpress SEO technique in which you can display different social media buttons on different posts, as relevant. This looks very useful, and I’ll be trying it shortly.
Mashable has 35 examples of corporations using social media. Skimming through these might give you some ideas you can modify for your own use.