Matthew Schneider is spearheading an effort to get the hashtag #ThanksPontifex (and it's multilingual equivalents) trending globally on Twitter at 07:45 Rome time, in February 28. On his blog, he offers further details, and gives local times around the world for when you should tweet using this hashtag.
For those of you in St. Louis or otherwise in CST, tweet around 1:00 p.m. on the 28th.
Also in the post are a bunch of sample tweets you may post, in addition to links to services that will allow you to schedule tweets in case you can't tweet during the appropriate time (due to work or some other circumstance).
Obviously, this isn't limited to Twitter; if you're on Google+, go ahead and use the same hashtag and see if you can get it trending there too!