<3 is a symbol for a heart.
Here is a screen shot:
It is unclear exactly why he posted it. Is it his last post on Google+? Is it that his misses his fellow Googlers? Is it his way of saying goodbye to Google? Is it that since Trump came in and he works under Trump that he is sad? Or maybe it is all of it?
Matt Cutts also confirmed in the comments area that he posted it on his personal account saying his corporate account is no longer active.
Forum discussion at Google+.