You can't offer to pay people who post their addresses, usernames, etc.
Accepting signups or requiring proof of participation for tasks requiring substantial effort (e.g. signature campaign signups, weekly social media campaign reports, media (videos, images, articles, etc.) bounty signups, etc.) via a thread posted in Bounties (Altcoins) is allowed though.
EDIT: Obviously, participating in such giveaways is prohibited as well.