See how many times a URL has been shared on Facebook
Twitter:
<a href="http://urls.api.twitter.com/1/urls/count.json?url=http://news.ycombinator.com" rel="nofollow">http://urls.api.twitter.com/1/urls/count.json?url=http://new...</a><p>LinkedIn:
<a href="http://www.linkedin.com/cws/share-count?url=http://news.ycombinator.com" rel="nofollow">http://www.linkedin.com/cws/share-count?url=http://news.ycom...</a><p>These APIs are marked private (as opposed to Facebook's) so use at your own risk.