Just how small are the tasks?
If you're thinking about tasks like clicking on a link or sharing something, your best bet would be Microworkers.
But Microworkers, for some people, is too expensive because it charges a few dimes per action.
I think your best bet would be to crowdsource like holding a contest, and everybody can participate by doing something simple.
The key here is simplicity.
Once people feel that they're not going out of their way to participate in a contest, you will have many takers.
So how do you publicize your crowdsource work? Just go to Facebook groups.
There are many contest pages there.
You can also try Reddit; the list is endless.
Just go to places on the Internet where people announce this kind of thing.
It's a numbers game, so understand that if you want, for example, a hundred actions, you might have to build enough attention for 1,000 or even 5,000 actions.
It's also important that you pay people.
If people do comply and win the contest, pay them so you can have another contest for a different project.
If you burn people and develop a brand for being a deadbeat, you're going to have to burn through many different accounts.
I'd rather build a solid crowdsourcing strategy by treating people right the first time around.