The song was written as a jingle for McDonald's commercials, based on a pre-existing German campaign originally developed as "Ich Liebe Es." Timberlake was paid $6 million to sing the jingle. Soon thereafter, the Neptunes produced a song based on the jingle and released it (along with an instrumental version) as part of a three-track EP in November 2003. A digital download EP with the same name was also released through the iTunes Store on December 16, 2003. The extended play included the title-track and a remix for all of the singles from Timberlake's first solo studio album, Justified. The song was also included on the bonus audio CD of Timberlake's first live DVD, Live From London.

Based on this passage, who says i'm lovin it for mcdonalds? Let's give stream of consciousness first:
The relevant information to answer the above question is: Timberlake was paid $6 million to sing the jingle.
So, the final answer is Timberlake.