With all due respect but this is the first time I've heard of Seafile, probably because Seafile isn't for consumers [1], SpiderOak is primarily a backup solution that learned to do sync, with its client implementations being weird, ugly and featureless and while I never tried Mega, I really don't think anybody sane can trust Kim Dotcom's Mega with any important files.
As anecdote I have zero acquaintances using any of these. Mainstream are Dropbox, Google Drive, Box.com, Microsoft OneDrive, Apple iCloud and Amazon Drive. That's already Top 6 and combined they probably cover more than 98% of marketshare.
[1] https://www.seafile.com/en/product/private_server/