I just spent a precious hour refiguring this out, so I thought I'd share for anyone looking the answer:
Zapier does not directly work with the Webflow CMS. You can use the Zapier connections to post new sign ups or work with your online store, but you cant use the CMS to push out from Webflow using Zapier.
What I needed was an easy way to update my Linkedin and Twitter social feeds whenever I posted to my Webflow CMS (this blog). There is a way to do it, but the documentation isn't explicit, so here it is:
1) Go to the "pages" item in the Webflow designer, and enable RSS. Webflow Designer > Pages > CMS (in Green at the bottom) > Sprocket. Scroll to the bottom and click the Enable RSS toggle.
2) Set up RSS so that it pulls in values from your CMS. For Description I used a custom field called Social as opposed to my standard Summary field. My reasoning here is that I believe the content of a post announcing your article is different than a normal Blog Summary used on the front page of your blog. Either is fine though.
3) Copy the RSS URL - its right where you flipped the toggle.
4) Re-publish your Webflow site.
5) Go to Zapier. Start making a Zap and select Rss by Zapier as the first step.
6) Have it trigger when a new update is posted.
7) Input the RSS url from step 2.
8) Connect your social media site of choice and fill out the fields as you like.
9) Test and publish.
One thing to note: If you are a free user, you have to setup a separate Zap for each social account. Its pretty easy, but not as easy as just making one, multi-step Zap. With Zapier you get 5 zaps in the free account, use them wisely!