I am guessing that the game is roughly 804 days old, as that is Ben's (sleuth admin's) age in the game.
The game has been tinkered with every so often but had one major change about a year ago with the expansion from one city to 4.
New objects are introduced at each new favour level you do, up to the point where your favour cases do not increase in difficulty. Also new items are added fairly often, like shoes people cried for. I am told there will be new objects added after the completion of the new FM trio, the doomsayers, which will only be available to those who have completed the three mysteries.
1. It had it's two year anniversary. Before going live, it had it's beta testing first. Then it turned to a paid site, which was about 2 years ago. The Sleuth admins detective is 804 days old. I am sure he was one of the first detectives created.
2. The game play itself had one huge change, where 3 new cities got added, new politics etc. Then there were a couple huge lay out changes. And further there have been changes larger and small. Sleuth doesn't work so much in terms of releases. Though sometimes even a small change, can make a MAJOR impact. Like when when we went from having the possibility to have two physical evidence contacts (barber, banker, etc.), to only be able to get one physical evidence contact and one other contact like the butcher, fortune etc.
3. There is no regularity on this. We have seen new items introduced a bunch of times already. Sometimes a lot of them all at once, sometimes a few. It doesn't occur monthly though, or even quarterly.