Yes, you should be able to do bounty campaigns for multiple ICOs on facebook. One ICO won't mind if you are promoting another ICO too on facebook. They should have no concern about it. Same thing you can do on twitter too.

You won't be able to do multiple bitcointalk signature campaigns because at a time you can have only one signature and they will ask you to keep the signature for a week without changing it.

As for the facebook ban, it is applied to paid ads of ICOs and cryptocurrency projects. They have no restrictions on posts about cryptocurrencies. So you can surely run the campaigns. I think this will give rise to Facebook campaign bounty as companies won't be able to run paid ads campaign on FB now.