First lets separate finding POT's on Seeking versus other sites.
I suspect that your Seeking profile is not generating as many unsolicited messages as the other site. And that's probably because the messages you get on the other site are 99% bogus; either not in your area, scammers, or GPS princesses. You will have to spend money to read them and reply - which is why that site encourages POT's to send massages to everyone. That's how they make revenue. Some here has claimed some success in getting a BCD conversion. But I'm not one of them.
Seeking, on the other hand has you buy a premium status for a set period of time and you can read and send as many messages as you want during that time. Note that POT SB's on Seeking do not have to pay to send or receive messages - but they do need to pay to use all of the filters.
Next, your question about who should/does message first; this comes down to testing various strategies until you find one that works for you. When you search for profiles, filters are your friends. You need a paid account to use all of the filters - it's worth it. Many here like to use a wide net approach - send out as many generic messages as possible to as many POT's as you can find and see who answers. Then weed out from those based on some criteria and reply to those who look most promising. Others, use a more targeted approach, crafting a message that is specific to a particular profile to demonstrate you actually read it, and to try to show your compatibility. And some, or at least I, wait to get a message first and then reply if I'm interested.
Whatever strategy you test and use, once you have a first message and a reply, you move into deep vetting the POT, and as quickly as you can, moving the chat OFFSITE to text, or similar app, or even video chat, to move the process along.
Finally, at this point, you need to decide if you want to close the deal and set up a BCD meet, or take a more cautious and slower path by setting up a M&G (meet & greet) first. And once you are off the site message system, the negotiation process is underway. There are lots of past posts about that...
Life is good
The Cat