It launched with only a few thousands of users or less, but Facebook is getting ready to test Graph Search more widely.
Users who signed up to be notified of when the new feature becomes available are already getting the option to switch to the new search engine, which is still very much in beta.
Regular users won't be seeing it for some time to come, the feature is only being offered to those that requested it themselves. You can still do that by joining the Graph Search waiting list.
If you've been waiting to play with the Graph Search, you should be getting a chance soon, if it hasn't been enabled for you yet.
If you've been selected, you'll get a notification and the option to turn it on then. You can postpone this if you choose.