For instance, a small company I noticed had logos of Dropbox, Evernote, Google, Salesforce, Box, Google Drive, MongoDB, Cisco as their "Ecosystem".
How do I notify the right authorities to let them know about unauthorized usage of logos?
I'll be honest with you - this is not a problem that needs solving, and it's not a valuable use of your time.
It reminds me of a recent Scott Adams article about who gets most offended by Donald Trump - and the answer was other people. So much frustration and negative media goes around in circles because of outragists fighting a battle, not for themselves but on behalf of ... other people they assume exist. Worse still, this gets in the way of actual causes where real people are impacted but don't get support or recognition because it's easier to run a "won't somebody think of the children" type headline.
It's fair to assume that Dropbox etc might have an issue with a small company using their logo without explicit permission. But do you know that's the case? With Trademark infringement, there's no 'authority' as such to notify - you'd need the rights holder to take action to protect their Trademark from being used in this manner. The fact that there's "a lot of companies" doing it would indicate to me that it's not important enough for Dropbox etc to worry about.
And if they're not worried about it, don't waste your valuable time and energy worrying about it on their behalf.
If that doesn't convince you, then here's how to prove me wrong: 1) Approach these companies - probably their marketing departments first, who will advise legal. 2) Ask how much they will pay for someone to identify and take down these unauthorized uses. 3) My guess is they'll spend something close to $0.
The great news is that if you prove me wrong, you'll be able to link to this comment about what a fool I was. I promise to take it in good humor.
As JacobAldridge points out, the "authority" in this case is the owner of the logo being used, so you'll need to find contacts at each company. On the Dropbox side, you can email brand@dropbox.com in general with questions about brand use. If the logo use is truly inappropriate, that team can decide whether to reach out to the startup.
Also in the blog, they have huge blogs posts publicizing big partnerships. They only have 1 or 2 blog posts with one of big brand name companies.