As any of your friends who have used a dating site would tell you (nobody uses one themselves, right? :-) ):
- There are lots of genuine female profiles on there.
- There are 10 times as many males as females.
- Women get swamped the second they are visible on the service.
- Women thus get to cherry-pick and reply to, maybe, one in a hundred, one in a thousand likes/pokes/messages/whatever.
- Each man then ends up sending a hundred/thousand such things before they get a single response. And that's just "on average", before you account for actual desirability, competition with the gym-goers, etc.
- If you can't win her heart (or other parts, as per her/your requirements) in the first few messages, she's likely swamped with others that can, and she'll move on quick.
- Because of the overwhelming response, woman come and go REALLY fast - whether because they are successful or they just get annoyed with the constant pinging of their phone. If you're not one of the first to contact, likely she'll disappear before you get too deep in conversation.
- Because women are there very fleetingly, and respond rarely, the men linger forever and try-try-try-again, just making the problem worse.
There's also spam and things but it's normally obviously just adverts for other dating sites, or trying to get you to talk off the main site from the word go. Filtering them (even if only mentally) is generally very easy. That some people are now openly marking themselves as polyamorous, up for adultery, etc. doesn't really affect much in the grand scheme of things.
Over the last 20 years, I've spent maybe 2 years in total online dating. Spam is a minor annoyance. Subscriptions get you more exposure (without a doubt, that's what you're paying for) but to be honest that's no guarantee of response whatsoever. Inquiry->Response ratio is pathetic. Response->Conversation ratio is poor. Conversation->date ratio is highly dependent on the people but often terrible.
Despite spending the vast majority of my adult life in serious relationships with fabulous people (so I obviously am not a leper!), dating sites have only ever resulted in discovering a small handful of friends in a similar position, a bunch of scary incidents, an awful lot of wasted nights out and no real success. Technically I (I mean, my friend!) found more love via Gumtree purchases than via all the dating sites put together.
However, dating sites play a lot of nasty tricks. OKCupid - if you're not paying - likes to pretend that you have a new 'like' almost every time you log onto the app and trap you into a recurring payment (you have to cancel every month). POF is full of spam entries that always contains the same keywords/photos (they aren't good at filtering them out even for the paying people) and the site/app are horrendous and don't even look similar to each other or have the same feature set! Match.com is STUPENDOUSLY expensive and really bad at matching, everyone I know who used it gave up really fast and then it's a bark to cancel. I'd comment on eHarmony but apparently there's some criteria on my initial questionnaire that I don't match which meant that after sign-up I literally wasn't allowed to do anything (not even see or search for a single member) and it just goes to a message that says they "can't help me" (or words to that effect! So very pleasant!) and it's impossible to delete the account or sign up on the same email again. I wouldn't mind, but it's not like I was rude in my profile or anything, apparently I didn't pass their "personality test"!
Outside of that, I'm sure the free/niche ones do all share data, but the big ones it's quite common to run into the same people on both sites because they've signed up to both, not because the data was shared. But you are giving an AWFUL lot of personal information and photos of yourself to a dating site, so you do have to consider what kind of reputation they have. Especially if you're female, these people are in charge of a huge list of single women, their login and messaging habits, their kinks, their personal messages, their address and credit cards, etc. etc. etc. You can't tell me that all those niche sites are controlling all that data from their techy guys, etc.
But fake-talking bot profiles just aren't worth the time. There are plenty of real people on all those sites. Anything that's spam/bot will be a third-party after your credit card number, not from the companies themselves to boost their statistics.