Since just before G+'s demise I have been posting equally across MeWe, YouMe, Tumblr, Facebook, Twitter, Hubzilla, Friendica, and Mastodon and whilst I see steady but slow growth on all, it is my MeWe groups that have grown fairly fast with my Technology and Gadgets Group just passing 8,000 followers.
So what I've noticed, and this may be true for anyone posting about a specific topic or interest, is that a social network that features groups by topic, tend to encourage quicker growth - it acts as a focal point to find what followers are interested in.
The other thing I have noticed is that many followers like to know what to expect in other words if the posts are about technology, that the feed stays on the topic of technology and does not bombard them with either cat photos or spam.
So even though YouMe Social does not have the base numbers that MeWe has, in the Technology category my Group still shows as having the same numbers as other top groups, whilst on Mastodon with its high base numbers my profile only has 445 followers and does not stand out anywhere (reason is much like Twitter my profile has to carry all my varied posts in one stream and there is no featuring my category). On MeWe my profile also only has 417 followers whilst the Technology group is 19x higher - which highlights to me the focussed interest being preferred by many vs general posts.
The same was true for G+ where one of my Collections had over 500,000 followers vs my profile with way less = focus, no spam, featured Collections and topics by G+.
My Facebook Page is also about the same age and only has 83 or so followers which either says something about Facebook users or the fact that Facebook has no directories for featuring categories by interests. My experience on Facebook was always that it tended to be more general (much like Twitter) or just specific brand or community pages (no real cataloguing).
Hubzilla, Diaspora, and Friendica suffer from a similar "problem" without any centralised directory by category of interest. Mastodon similarly but one thing that differs with them is the concept of many nodes themselves being a focus are by a specific topic eg. women, engineers, geeks, etc but still no real directory by category.
I can see one day, when I have time, I'm going to recreate the old search engine style where we had sites categories and grouped into a directory (remember Alta Vista and Yahoo?), but instead of indexing websites I need to index by category of interest across all social media platforms...