Hidden away in a recent update was this blunt announcement:

Leaderboards are going away. Sorry. The rank-whores, modders, boosters, cheaters and assorted scumbags have so polluted the Leaderboards that they were basically becoming an official list of America’s idiots anyway. There are a few genuinely brilliant players up there, and to them we apologize. To the rest of the folks on the Leaderboards: Tough. Enjoy your hard-earned anonymity.

Hardly surprising and I commend them for their forthright adamance on the decision. PGR2 suffers a similar problem and even online flash games are plagued by cheaters. Every one of them scum.