I have very little to add to this, other than to say 1) it's a rare incident of Twitter blowing up over what a foreign news service says, and NOT immediately finding out it was a hoax, and 2) Popes don't abdicate. There were whispers and nudges for John Paul II to abdicate as he was ravaged by Parkinson's, but he didn't. No Pope has left office voluntarily since 1417, when Gregory XII quit as part of a deal to end the Western Schism. There were rumors about the 86-year old Benedict considering this, last year, but it really is unheard of.
I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering. However, in today’s world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me. For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of Saint Peter, entrusted to me by the Cardinals on 19 April 2005, in such a way, that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a Conclave to elect the new Supreme Pontiff will have to be convoked by those whose competence it is.
And on February 28, the Santorum for Pope campaign can begin.