I'm no influencer, how can I run a campaign myself?
You don't need to be an influencer, you can run your own campaigns with your terms with your fresh new sm accounts.
I don't have a precise budget yet, I plan to start with a few hundred dollars, could scale up if the response is positive
the quality depends on your budget as well as duration of the promotion. you will definitely have to decide for yourself.
My target is Ethereum users, and I'm only aware of Tweets and Facebook posts, what other options are there?
You have many options here, I will send you PM with some of the ideas.
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Crypto related blogs website , or youtube video
Be remind that facebook have a problem in posting in many different crypto related facebook group. i have many friends doing that and they experience ban for posting for 3days or more.
So facebook is not a good choice, unless the people you hired have thier own crypto people target in his main account.
I have a FB account and also moderate few crypto-related groups there, I did not see any restriction to post real posts. Spamming with scam "opportunities", promote investment platform it is always risky to be banned. In this case, it's not OP's problem.