Sure, I would appreciate all suggestions and will reward anyone if we implement their idea in the future too.
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OK. What I would really like to see is something like the 'Birds' service (https://birds.bitcoin.com/about), so basically paying/getting paid for re-tweeting. The problem with 'Birds' is that they operate on-chain, making it pretty much unusable for anyone with less than 10k followers.
Freebitco.in already has large userbase and off-chain payment system, so it's a massive advantage. You could charge even 10% commission and people still will be happy to use it. You already have a massive crowd of people looking to earn small amounts of btc (I'm sure a lot of them have twitter accounts), and there will be people willing to pay for retweets - it's much easier and likely cheaper than running a standard "twitter campaign" (often done manually, as seen on this forum), also it seems a good way to promote ref links etc.
One obstacle to overcome would be filtering out people with thousands of fake followers. But it's doable, you could check the audit score at registration or at withdrawal attempt, set the minimum allowed score or only pay for real followers. Hopefully it could be automated.
Other idea: Staking PoS coins for a small fee. Something that JustDice does with CLAMs. You could allow users to convert their btc earnings to PoS alts (CLAM, NXT, in the future - ETH).
You could also integrate ShapeShift for deposits/withdrawals (it would probably only work for instant withdrawals though). From the users' perspective, it's always good to have such option, even if the fees are rather high. Afaik, it's easy to implement and ShapeShift pay affiliate commission.