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I think promotion would be my favorite, because it's the most unobtrusive one, but maybe we need more...
If I remember well, advertising was already mentioned in the whitepaper, so it's nothing strange and hopefully will improve users' rewards too.
I think is possible to create some creative and effective way to advertise, possibly avoiding old-style banner ads.

Yes Ned was talking about it recently too.

Yes but I think we need other ways of advertising too.