Universal did announce their 20th Anniversary via Twitter for sure and usually what appears on their Twitter is also on Facebook. So it hasn't been totally forgotten about.
An intersting note: Disneyland Cast Members (or Cast Members who go to Company D) get a reduced price ticket to USH. Interesting that competitors will give discounts to each other! Of course, CA is a whole different animal than their FL cousin!
As for annnouncements, I really think Disney dropped the ball (no pun intended) on EPCOT's 25th
Lew Wasserman was still alive back then, when Universal Studios Florida was owned by MCA and Rank Organisation (before Universal went through its Vivendi, uh, adventure on its way to NBC and before Blackstone Group emerged and obtained Rank's assets). But I doubt that Universal had much more money then - the park had just opened and had little income.
No, Universal needed a bold move to save face and win back some customers. Giving out free tickets didn't cost them any cash up front and probably ended up exposing the park to some fresh customers in the long run, thanks to people like me handing out free tickets left and right!