Absolutely! I've been to Thailand twice and have tonnes of female friends who went there solo. like every other place there are some basic things you do need to take care of and be aware but Thai people are a friendly lot, you'll be fine :)
Yup, it's very safe... recently been there with my sisters.
apart from a tuk tuk driver, who seemed very weird , we didn't encounter any problem throughout our trip... rather the people are sweet, humble & helpful..u shld be fine with ur pals :)
Yes it is completely safe to travel. Places like krabi, phi phi island and Phuket are famous for their beautiful beaches and night life. You can view my post on my recent visit to Thailand where you might be able to get an idea on which places should you be visiting in Thailand.