ActivityPub is a W3C standardized protocol for interoperable social networks used by Mastodon and others. Bluesky is a social network that has announced plans to enable federation using its own AT protocol in lieu of ActivityPub, but is still entirely non-interoperable at time of writing. However, all post data on Bluesky’s network can currently be easily accessed through APIs offered by its central server. Pinhole makes use of these interfaces to fetch the posts of a specific Bluesky profile and make them available as an ActivityPub actor that can be followed from compatible networks.
Pinhole currently supports bridging top-level posts, including attached images, as well as self-replies (threads), though not replies to other people. It also mirrors the profile description, display name, and avatar. Due to it being a one-way bridge, the posts it mirrors can be read, but interactions such as likes and replies will not be visible to the original author.
It should be noted that, while the ActivityPub half of this bridge is relatively by-the-books, the Bluesky half is not an implementation of the AT protocol at all. Instead, it relies on known patterns in current Bluesky API URLs and lightweight response parsing. As Bluesky makes changes to its architecture, Pinhole setups might break. I will probably update it to keep it working for as long as I personally need it, but in the long term (especially if Bluesky sticks to its plan to provide federation through the AT protocol), the community will want to move to proper bi-directional bridges built by other people. If this is an area of interest to you, you probably want to follow the work of Ryan Barrett.