While that may be part of the reason, it's not the only one. Here's what I picked up as a complete nobody (I have no deviantart, tumblr, facebook or anything) :
An easily likable story : As I said, I came from nowhere but got 574 subs the first month. I believe that my thumbnail was attracting enough and my story is sort of mainstream (a bit, I guess), using familiar tropes that makes it appealing to a large audience. I did not write it that way for that reason of course, I'm my own audience, it's just a happy coincidence, so don't make your story mainstream if it means destroying it just for the sake of gaining subscribers.
Being Staff picked : I had the chance to get staff picked about a month after begining which got me 200 subs in a day and enabled me to stay featured on the main page for several days after that.
Being featured in the spotlight : I haven't had the chance to get there but I suppose it's the same as being staff picked, only better =)
Appearing in the daily snack : One day I had the good idea to post 4 pages in one go instead of just one, which got me in the daily snack and I gained 2000 subs in 24h, that was crazy. I was really happy to suddenly be able to reach out to the readers that only used the app and couln't find my comic any other way... So my best advice would be to post several pages in one go when you begin with your comic (if it's a long-form story, not talking about gag-a-days, those can get you into the DS anytime) I know the staff is more likely to feature the beginning of long-form stories so that the new reader isn't completely lost.
Posting thank you drawings. (It contributes more in a steady income of subscribers rather than a big boom but it helped a lot at the beginning I suppose) I know a lot of people discovered my comic via the TY drawing I keep posting on my readers' wall, (though I'm completely behind on that, with that 2000-subs-in-a-day thing). When I keep seeing the same drawing on everyone's profile, I inevitably check the comic out since a lot of people seem to love it... (Though I post my drawing privately when the readers has a hidden reading list). Note that this is a side effect, I don't post TY drawings to advertise myself on other's wall but more as a way to open communication and encourage people to leave a comment.
One of my friend who also came from nowhere posted the first four pages of her comic and immediately got staff picked and featured in the Daily Snack. Bam ! More than 3000 subs on that first glorious day.
Anyway, sorry if this feels like I'm bragging, it was absolutely not my intention, but I just wanted to give the perspective of someone who got a lot of subs pretty quickly without bringing their tumblr army with them. Pure luck played a big part in that too I guess.