All silos are not equally silo-y.

For example, Twitter is a silo, but I can reliably point to a tweet from outside Twitter, and you will be able to read it even if:

• You are not logged in to Twitter.
• You are not following the person who wrote it.

However, if the user has blocked you, you won't be able to read it.

So that's mostly like the open web in this regard.

Facebook on the other hand is also a silo, but you can't reliably point to a post, because unless you're willing to research the settings for this post and the settings of the user, it's so hard to say whether or not people will be able to read it, that I usually don't bother linking. For example, here's a link to a public Facebook post. Can you read it? I don't know.

Some sites we don't think of as silos, are as hard to point into as the most restrictive silo. For example, news sites with complicated paywalls. Some paywalls are designed to be porous, meaning if you point to them from a social media site they let you in without paying. I find that's hit or miss, and none of them view my rivers as exempt from the paywall. So I tend not to link to stories that are behind a paywall. 

The point: we could come up with a Consumer Reports-like rating for all sites that indicates how much they participate in the open web at a content level.