This question already has an answer here:
I see "share this link to earn the publicist badge"
Where exactly do I share the link?
Take the link from the
This could include Facebook, Twitter, any other social networking site or your blog, for example. But could extend to non-web methods, maybe sneakily write it on the whiteboards around your school / Uni / workplace, directly tell others you work / learn with, give it to with your mates down the pub, and so on...
For further information on the Publicist badge (and its family) have a look at the List of all badges with full descriptions.
You must share the link to any other site from where the traffic gets directed to SO