In all the rush to sign up to Twitter and Facebook people seem to have forgotten the basics of websites - 'Content is King'.
The content of your Home Page is just so important, it has to contain what you do using the phrases that people are likely to use to search. Of course this is a moving target but if you want the search bots to come visiting regularly you need to change the content as often as you can. Plus you have to ensure the bots can actually read it so check your source page as if the text does not appear there the bots won't find it. Which is why a Twitter feed is great for updating the site for the reader but of little use for SEO as the text is not available to the bots on your site only on your Twitter page.
Then there are the backlinks - the links into your site from other online spaces which gain you brownie points with the Search Engines. This is where Social Media plays a very easy role, Twitter is very good at bringing new people to your site, so to is Wikipedia but you need to find just about any and every site that could list your URL.
Once all this is in place you can look at a Pay Per Click solution making sure the keywords are chosen so they appear on the relevant search term pages - too broad a term will be expensive and may be counter productive. The Long Tail theory really does work here. Ensure the text of the ad is carefully crafted and the link goes to a landing page that contains all relevant information and call to action.