Both work real well.
On page
SEO is not rocket science . Learn it ... live by it .
Social bookmarks is a compliment to your on page. As an off page
SEO source ... make sure you are tagging the same keywords on the social sites as you are taging for your on site.
The search engines do like it when everything is relevant . Wow ... only two post and I have used relevant in them both. Yup relevancy is just that important .