That's almost exactly how I use both of those sites. ^^; I retweet constantly on Twitter, and although Tumblr is what I consider my 'main' site now, I only average one post every 10 days, more or less.
Anyway, I highly doubt you're the only person who doesn't 'get' social media...I think the vast majority of people who are super-popular on social media are either (a) lucky, or (b) have an established form of fame that draws followers in automatically: maybe they work for a famous organization, or they're part of a famous group, or they're literally a movie star. ^^;
Fortunately, (a) luck can come to anyone if they wait long enough, and they may not have to wait as long if their account is a pleasant place to be.
That's the one form of social media advice I would be confident giving to anyone: just make your account a pleasant place, especially for you. Fill it with things that are fun and things that make you happy, so that no matter how long you have to wait for your number to come up, at least you won't be miserable.
Now, if growth is important to you, I'd also advise you to make sure your original posts-- the things you want people to pay attention to-- stand out in some way, and are easily accessible.
For example, on Twitter, even though I retweet 95% of the time, the only image posts on my account are my art. If you visit my profile and look through my photo gallery (or whatever it's called) you will ONLY be seeing my drawings. No memes or screenshots, and very few actual photos. Just art.
And on Tumblr, I just limit my reblogs, of which there aren't many, to (mostly) text posts. So if you're scrolling through my blog and you see a cool drawing, the odds are extremely high that it is mine. ^^
Basically, make sure that if the average stranger is considering following you, it is as easy as possible for them to see what you have to offer and make that decision.
That's it...everything else is just various forms of 'the whims of the algorithm'. :T Stuff the site developers design for you to pointlessly and endlessly chase after, and are honestly just a waste of your time unless you're already famous.
If any site's suppression algorithm is so ruthless that you are basically invisible unless you actively play their game, I'd say just don't use that site. Although I think most sites are willing to give you a chance-- they recognize that the vast majority of their users don't even realize that there is a game, let alone intend to play it. ^^; So I don't think there's any harm in just hanging out and biding your time.