I have an existing install of XF and successfully migrated all my data from vBulletin. I have users who have boards on free services like Boards.net or Freeforums. Is there any way to migrate data from boards like these - specifically threads and replies?
Most free forum sites don't give access to the actual database or charge you a large fee to export so you may have to get your content with some sort of custom scraper unfortunately. The best part of self-hosting your forum is that you control your data. For example my current forum started on phpBB2, then went to phpBB3, then XF 1, then XF 2, and if I needed to, I could import it to some other software someday. Basically self-hosting allows you to keep your forum through software changes. With free hosted forums you're kind of at the mercy of the host - for example before my phpBB2 board, I had an ezBoard, which got sold to Yuku, and then Tapatalk. Tapatalk uses phpBB3 as a base but the database download is disabled so I can't just import those ancient posts into my XenForo that I use now. Your options are basically:
Try to get the old host to give you an export (usually involves paying them and your export may or may not be importable to XF)
Use a custom scraper to download the old forum into some import-able format for the XF to import (requires paying a developer)
Manually copy the content you want and post it on the XF (lot of work)