There's a new command in the popup menu for each post in v0.90w of the editor that allows you to set the Twitter and Facebook metadata for each post.
The three bits you can change are the title, description and image.
This enables you to have a different description for the post in the metadata, having it be a summary for the post instead of the full text which is the default.
You can have a different title, which you might want if you want a clickbaity sort of title, like "The pope met with Zuck and you won't believe what happened next!" Just kidding. :grin:
Finally, you can supply a link to a beautiful image which is displayed when Facebook and Twitter show people your post. This was the main reason for doing the feature.
If you started using 1999.io after Sept 4, you don't need to do anything to set it up, the new default template already has the new macros.
If you're upgrading an existing site, look at the Facebook metadata and Twitter metadata sections of the default template, and do what it does.
The new macros are titleForMeta, descriptionForMeta and imageForMeta.
Create a post.
In the editor, click on the down-pointing triangle and choose Set metadata from the popup menu. Screen shot.
Enter the title, URL of the image and description in the appropriate places.
If you leave an item blank, the default value applies.
Questions here or on the 1999-user mail list.