Ponga "previews" the contents of the first comment in a selection when you, contributors, and other viewers explore the selections in your picture. Moving a mouse over a selection will cause the content to "preview" or "unfurl" much the way content appears in Facebook when you post external links.
What content in a selection will previews depends on the media type, and how it's entered into selections. Generally, media takes precedence over plain text so that your viewers get the most engaging possible experience from exploring your picture.
Embedded content like recordings, identified faces, or video will preview first. If none of these are available, then text of the first comment made in a selection will preview. If, as the owner of a picture, you'd like to change what appears in that first selection, you can delete the first comment and the contents of the second one will preview.
Preview content without the link
There are times when you want to preview the content of an external link without distracting your viewers with the link itself. The simple way to do this is to paste the link, let it embed, then post. Any link added into a comment without additional content (such as a written comment to provide context or background) will appear by itself without the link visible. Clicking on the media often provides a link to the original site.
Which comment previews with mouse-over.
If you want to change which comment in a selection appears first and thus previews when you mouse-over the selection, you will need to copy/paste and delete to move comments around.
Control with the link
What if you want to include a link, but not have the contents of that link preview in the comment at all. Power users of social media will know that you can control the previewing of content by selectively entering the protocol identifier (e.g. http:// or https://). Links included without the “http…,” will automatically link, but not preview their contents in a comment.