According to a study by folks that track this sort of stuff, Danica Patrick is the #1 Celebrity Endorser when it comes to Social Media Buzz. The study looked at the number of mentions of a brand along with the celebrity on Twitter over a 30 day period, and came up with this list.
#1 Danica Patrick / GoDaddy 12.72%
#2 Rihanna / Cover Girl 3.19%
#3 Lebron James / Nike 1.73%
#4 Alec Baldwin / Capital One 1.36%
#5 Jennifer Aniston / Smart Water .32%