I have earned most from offers. I get very few surveys (about two or three per week on average and even for those I'm often not in the target demographics), but I assume many of the offers are targeted much more globally.
Tasks are another good potential source of income (assuming you have no trouble completing tasks that are in English), but I still haven't managed to get enough tasks with test questions to qualify for levels 1 to 3 on CrowdFlower. Currently my accuracy would be enough for level 2 (and I've been extra careful lately with the test questions to hopefully improve that), but it's been a while since I've received any tasks with test questions
Tasks also come in many languages: just recently there was a job with huge bunch of tasks in Japanese and the pay for those was really good compared to most tasks I've seen/completed. That really made me wish I hadn't dropped my Japanese studies all those years ago...
So, knowing an extra language or two can be of help with the tasks.