The issue reported by Dharrison returned. This time, though, it was not disappearing of its own for a few days in my account. For the last 3 days or so, my list of Friends in the lef-hand panel of all webpages had been empty. (I currently have 135 friends.) Tested in Chrome, Opera, Firefox and Internet Explorer. Restarting the browsers, logging off and on multiple times did not resolve the issue.
The issue was only resolved by launching the game window. In the game window, the Friends list displayed correctly. What is more, I found 3 unanswered invitations waiting there for me, although I had previously visited the forum webpages a number of times in the last 3 days. (I just didn't have the time to play.) So, the webpages failed to notify me of those invitations. (Having the option to receive an email whenever someone invites you for a game or requests friendship would be nice, too. Facebook and last.fm do this, for example.)
After opening & closing the game window (even without launching a round), the Friends & Invitations panel on the webpages recovered, too, displaying both lists properly now.