Thailand is a large country and if anyone thinks the majority of the population can speak Thai, then they have never been outside of Bangkok or the tourist areas.
In places like Isaan, about 5% of the people can speak enough English to give you a simple answer to a simple question and the same is true for the 3 most southern provinces of Thailand.
North of Chiang Mai/Chiang Rai the answer is the same. You cannot find anyone speaking English.
Go to resorts such as Hua Hin, the receptionist will speak English and the rest of the working hotel staff, none at all.
The security guards at the Sofitel Hua Hin can say good morning and that is it. Cannot even give a simple direction.
In Bangkok, get outside of Sukhumvit Road and you will find the majority of people do not speak English.
Go to the Thai section Rachadapisek and see for yourself, few people can speak English. The official Apple Store at the Esplanade Mall has 10 employees, one can speak enough English to help you out, the other 9 always run to get the last person.
When I say speak English, I am talking about being able to hold a simple conversation, answer questions in English, give directions, etc.
Any one that answers that the majority of the Thai population can speak English, prove it? You won't find one statistic to back up your claim that the majority of Thai's can speak English because it is not true.
As Bob said, people can say hello, where you from, what's your name? Because this is all memorized but give them a