Nowhere is this clearer than on Linkedin. Whenever I log in repeatedly and am active on this site for a while -- particularly in the discussion groups -- I get connection requests in the days afterwards. Makes sense, of course, since I've just put my profile in front of more people.
Also, on the right hand side is a little section that tells me how I am fairing in searches. If I have not been active, this says my profile appeared in less of them than it had before. If I have been active it says I've appeared in more of them. Basically, it goes up and down like a yo-yo according to this activity. Now, it is possible that this could be due to an unrelated factor, and the correlation is just coincidental. But I seriously doubt it. I've observed this phenomenon several times since I joined the site.
Something similar happens on Facebook. As we all know, likes are the currency of that site. If you don't keep reaching out to other pages by liking and engaging with them, as well as posting updates to your own page, you almost disappear from view entirely! You'll notice a massive drop in the rate at which your page gains new likes.
That said, you'll still get a few trickling in during the ensuing weeks. And the longer you've had your page up, and been active with it, the frequency of these likes during your inactivity gradually increases.
So on Facebook, as with Linkedin, there's the immediate, human effect. And there's the long term, residual, algorithmic effect. Since you've racked up more social media "cool points", the network makes you more findable in searches, etc.
Now, unless you worked super hard for ages to build your followings on these two networks -- as well as paid for ads to lift your profiles even more -- I doubt you'd ever reach a point of critical mass after which your followings kept growing without any more effort on your part. If you want them to keep building inexorably you're going to have to keep returning to and participating in them.
That said, there is one social network in which the momentum you build up from frequent activity seems to bestow benefits for a very long time indeed. And that's Twitter.
I have several accounts on Twitter in different niches. A couple of them are a few years old now. On both of these I still get new followers every day, even when I've been inactive for a while. Now this doesn't necessarily mean that my networks keep building in size because people will be unfollowing also, particularly if it's a while since I tweeted. Still, it's interesting that this occurs. And it doesn't happen on my newer accounts.
Obviously this has something to do with the fact that in each account my profile is in way more locations on the network such as in other tweeps' followers and following lists. Also, being so established, and strongly associated with certain keywords and hashtags, the internal search function of Twitter is showing my profile and tweets more often.
This effect seems to really have kicked in lately for my politically oriented account, which is heartening. Not only is my network building more consistently than before, with the same effort. But my tweets seem to be findable for far longer than before. These days, I quite often get retweets and replies for tweets that I posted several days previously. This effect seems quite marked. And it has only been really noticeable this year. So it seems to me to be mainly the result of an algorithmic factor.