which include:

Search demand
User intent
Competition
Budget
Timeframe
Goals
Profitability
Scalability

click here for details :
http://searchenginewatch.com/article/2325344/8-Deciding-Factors-that-Influence-SEO-Keyword-Prioritization

commented: please begin a discussion rather than lists and links -2

Matt Cutts said. "A lot of the times if you land on your site from a search result, even if they've been in business for 15 years, 14 years, sometimes they haven't updated their template or their page layout or anything in years and years and years. And it looks like, frankly, a stale sort of older site, and that's the sort of thing where users might not be as happy about that."

which social sites is best for SEO

Facebook
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