I mentioned that I was working on secure online backups on my blog back in 2006, and by the time I was ready for beta testers I had a long list of people who had emailed me to express interest. Once I had all of those people testing tarsnap and still wanted more testers, I posted again to my blog and submitted that post here and to reddit, which brought me many more testers. Recently it seems that every time I've posted to my (personal) blog there has been a spike in signups; presumably people come to my blog, see my earlier posts about tarsnap, and then get interested.

I don't know how useful my experience is to you, since the sort of experienced *nix users who appreciate tarsnap are generally far more cliquey than "news junkies" -- but you asked for my experience, so here it is. :-)