On Probuilds it says Muramana, but I think that includes Manamune too. U.GG also shows it as Manamune, so this is entirely possible.
There are at least 4 in the last 50 games on probuilds.net. Not sure how or why U.GG, as I've never used their recent builds section before...but the list of recent games is different than probuilds.
That being said, you missed a bunch. There are at least 10 in the last 120 games (including both Muramana and Manamune) on U.GG, and at least 4 in the last 50 games on probuilds. You might want to look again.
EDIT: It is actually really weird that Ghostblade isn't in the top 6 on probuilds though, considering it's built nearly every game. Not sure why that is. Something fucky's going on. Even so, U.GG says 11% as well, for Manamune.
EDIT 2: It's also not necessarily the case that you'll see one approximately every 10 games. They could be clumped together. Like, one week there might be 5-6 of them, then none for the next 3-4 weeks. There are a lot of reasons this could be the case. Meta champions on the enemy team (that then change one week), one player who likes to build it, etc.