That keyword is very competitive. Even you build so much quality links you will have a hard time and you will spend a lot of time and bucks before you will get on the 1st page.
Add Unique content with your targeted keywords and try to get back links from those websites which has good rankings for your targeted keywords.
Build quality links to your site pages from other well ranked sites on Google. Make sure you provide quality content that have something unique to offer and that have keywords or key phrases people might search to find your site in Google. Optimize you web site pages by making sure your top keywords appear in your title, meta tags and content. If you sell products, give something away free (The word "free" is one of the top most searched words on the internet). Keep a track of your listings/ranking in Google and analyze it periodically. Some google tips on this: Indexed Pages: To know which pages on your site are listed in the google and to see how they will appear to google searchers type site:www.yourdomainname.extn in the google search box. Link Popularity: To keep a track of sites that link to you (and also see their google page rank), type link:www.yourdomainname.extn in the google search box.
Well, in order to increase the PR of your website in the Google, you have to increase the CTR (click through rate) of your website if the content of your website is good and unique then the CTR of your website will increase and when the CTR of your website is good then you will see your site in the top rankings...
Make a useful content on your website including your targeted keywords. Use other off-site optimization techniques like - Social bookmarking Link Exchange One way link development Forum postings Blog Postings and Commenting
Social Media Agency is a very competitive keyword. I guess it will take a year before you will get your desired results.
You will have to concentrate on social bookmarking sites. Keep posting good and unique content and bookmark them in high ranked sites. Try to get one backlink a day (a one way link if possible). Link exchanges normally do not help as the links get diluted. Frequently keep commenting on forums like DP and also on the blogs with your same niche.
PR is determined by how many backlinks from other sites to your site. But not all links are counted by Google.