After some testing, I think I've cracked it.
Initially, sites seemed useless. Advertising does nothing, on sites or on scenes, so sites are completely incapable of generating fans, and without fans, you get no subs. So sites are incapable of generating their own revenue.
So starting off a new save, use ads to get around 15 level 1 girls and churn out scenes, so you can post every day. Don't be afraid of posting too high a wage, as that only seems to affect the initial scene. The game will generate a salary for each girl based on her stats for every scene after that. Don't bother with advertising scenes, it does nothing. Chemistry does seem to have a slight effect, so probably try to hire the type you selected in chargen. If you notice the girls you hiring disappearing from the menu, just go to the city screen and go back. I didn't test if stress does anything, but it's easily managed by browsing the net, so it doesn't really matter.
Once you're posting every day, it's a loooong grind for fans. Your priorites in the shop should be the laptop, so you have more time in the day to go improve your stats, and cameras, lighting equipment, and home improvement, to increase the quality of your scenes. If you want to min/max in a porn game, don't buy any of the charisma increasing items until your charisma is at 30, as they can increase it beyond 30, and although it might be a placebo, it seemed to give a noticeable improvement to pick up line success, which is of questionable value, since ads are easier to get girls from anyways. Now then, you're in for some repetitive as fuck gameplay for a while, because you need a fucking high fan count, and without cheats, you won't have enough money to get the items to really jack up the quality of your scenes for a long time. I did a lot of the testing for site profitability at around 40k fans, and it didn't seems great, compared to just churning out scenes. Also, Sites pay out every 30-ish days, meaning they don't give significant income individually, even with a high sub count
Using multiple sites does make it a little bit less massively skewed, however. The ratio of subs to fans seems to be around .0075 subs per fan. However, each website seems to calculate that individually, and growth rate is affected by trends, which you can only see if you have a personal assistant. So if 1 site gives you 1000, 2 sites will give you 2000, and so on, there is no curve, just a flat increase. Sites don't suffer from using scenes from only one girl, either. So use the highest beauty girl you have, for every site, and upload 1 scene per site every few days. Don't worry too hard about happiness, it takes about 20 days after hitting 0% for the sub count to start dropping, and as long as the sub fee isn't too high, it'll even climb right up until it drops. High sub fees do affect site growth, by the way. Low sub fees do seem to boost growth, but not meaningfully.
Fan count doesn't seem to affect scenes either, only sites.
TL;DR
Once you have a lot of fans, how many is uncertain at the time, use as many sites as you can afford, and use the highest quality scenes you can, even if they're from only 1 girl. You'll have to use scene uploads to get fans. NEVER use advertising on scenes or sites. It does nothing except drain money.