<a href="http://www.facebook.com/sharer.php?u=URL&t=TITLE"><img src='facebook.png' /></a>
The additional title / 't' parameter is optional.
<a href="https://twitter.com/intent/tweet?url=URL&text=TITLE&via=TWITTERHANDLE" class="xml plain">><img src='twitter.png' /></a>
Note the additional TWITTERHANDLE in the 'via' parameter will mean your twitter name will show up in the tweet using "via @yourname". Title is optional.
<a href="https://plus.google.com/share?url=URL"><img src='google-plus.png' /></a>
<a href="http://www.linkedin.com/shareArticle?mini=true&url=ARTICLEURL&title=ARTICLETITLE&summary=ARTICLESUMMARY&source=ARTICLESOURCE"><img src='linkedin.png' /></a>
ARTICLEURL is the only required parameter here, summary, source and title are optional and must be url encoded.
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