I emailed outbrain, plista, revcovenent last Thursday. Still no response from any of them now. This is very unprofessional at least in my books. Is this normal?
Did you just email them or created and account there too? It's true that emails often get unanswered, but once you create an email and follow up with them that way, it usually works. Linkedin can help a lot too. And it's always good to see if they have a rep here and PM them 
Where are they based? 1st and 2nd August is a civic holiday in some parts of Canada, so maybe it's been only 24 working hours that they didn't reply to you.
To get anywhere with Outbrain and Revcontent you'll either have to ask someone for a rep or find one on Linkedin. At least that's how my experience went.