Those familiar with microblogging no doubt ask themselves: what sets postActiv apart from other microblogging services and software, such as Twitter, Mastodon, or Facebook. There's a number of things:
Unlike proprietary software like Twitter or Facebook, the source of postActiv is open and freely-available. Licensed under the AGPLv3, the code of postActiv respects your freedom and allows you to make whatever changes you might desire to the code, and run it under whatever environment you want. You don't have to worry about complicated and expensive licensing schemes, and server operators or developers can extend the software in whatever way they wish, knowing that the Four Freedoms are respected.
Sites running postActiv can communicate between each other, allowing you to operate your own social network curated exactly the way you wish, all while not having to worry about the traditional forum software problem of a small and niche community, since you can easily follow and discuss with people from other server running postActiv. It can also communicate with servers running Friendica, Hubzilla, GNU Social, StatusNet, and Mastodon.
(Mastodon does not follow the OStatus standard strictly and thus there are a number of inconsistencies and issues with Mastodon posts, but you can still communicate freely with people running Mastodon.)
Many other federating software have a plethora of issues such as loss of messages in transmission, lack of updating profiles, not finding all the messages in a given conversation - postActiv strives to ensure that the network is sound and solid, and that such problems are minimized. It includes a variety of checks and balances to even out federation issues and ensure you can communicate without interruption.