Seems like all airlines periodically have these "100% bonus" periods. You just have to follow the different carriers. Subscribe to their emails, follow them on social media, etc. And follow the FT forums, of course.

At the end of the day, the "base" rate to buy FF miles is a little over double what most people find them to be worth. So the 100% bonus essentially brings the price roughly into line with the value a skilled user could redeem. I personally look at the offers as a way to top-off for a large premium-cabin award, not a primary source for miles.