I am using two indicators :D.
The first one is the number of Gaiscioch members currently playing. The number has been steadily decreasing since launch and it seems to be kinda stable now at around 545 playing. Which is still a big number but it was only about 40% of total members playing at launch.
The number of reddit/r/elderscrollsonline subscribers. This one didn't decrease quite as sharper as the number of our members playing ESO. Because the Gaiscioch number-playing is quite an accurate number of guild members playing, you have to check-in every week to be counted. Whereas the ESO subreddit sub number is much less so, because you have to actually unsub from it to not be counted. But the trend still kinda hold. I see the number bottoming out towards around 40,000, never did it break 40,000, and now climb back to > 40,100.
So I am venturing a prediction that both the ESO subs and numbers of Gaiscioch members playing might have bottomed out around this time (07/18/2014) and they might actually go up from here.