Elder M. Russell Ballard was the first apostles to write a Tweet back in May, and shortly after, the other apostles (with the exception of President Monson) joined Twitter.
Today, LDS Living has confirmed with Church Public Affairs that President Monson has joined as well--and he's already written his first tweet. Check it out:
Love is the very essence of the gospel, and Jesus Christ is our Exemplar.— Thomas S. Monson (@ThomasSMonson) September 25, 2014
You can "Follow" the prophet by clicking "Follow" in the post above. Follow the other apostles at their official handles: