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For SEO strategy, you can join social media sites like Twitter, Facebook, Google+ and Instagram.
It depends on what you are going to post there. For me FB and Instagram are most effective.
That is true, but compared to Facebook it's sadly far too small, at least here in Germany.
I suppose it depends on GEO because there are lots of social networks besides FB, twitter, Instagram etc, that popular in different countries
In my opinion Instagram and Facebook are more efficinet platforms to drive target audience.
There are many tools that help to automatically attract them.