I’m struggling to find where I heard about this, but if you post to Twitter (or I guess it’s X now) and tag @Sync, they should get back in touch with you and offer you a bonus 10GB for the positive outreach.
Since I don’t know about where I heard about the offer originally, the next best thing might be my post which Sync responded to as evidence of the bonus. Along with one or two other bonuses which one may have been a referral, I’m at 17GB on the free account which is pretty decent, and certainly not as burdensome as the referral process one has to go through with Dropbox to grow the free tier there.
They’re a great service from the time I’ve spent with it and worth a go.