Salman was popular. But not with so much margin. Popularity changes over time. Now all the movies of Salman are ATBB, so he is more popular now. A while back SRK was popular when his movies were doing well while Salman was going through lull phase.
Having said that Salman has always been more popular than SRK. SRK was more popular with NRIs, foreigners, girls and city people, this are the people who participate majorly in these type of surveys. While Salman was more popular with small town and villages(which constitute majority).
Aamir and Saif have never really matched the other two's popularity..