It seems common to social media sites that they assume once a member or subscriber, thou shalt not leave; though a reputable one should not stop you.
If a paid-for site it may be even harder, especially where such sites use an intermediary payment company. Whilst that has privacy and security advantages, it makes stopping the account more difficult.*
If nothing else you should be able to at least edit any profile you have placed on it, replacing all the information with a simple "I am no longer a member", or even just blank. Delete anything else you can.
If there are members with whom you are "friends" to the extent of wanting to keep in touch, you might be willing and able to use Private Messages to establish an ordinary e-post or telephone exchange.
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I fear costly up-front fees or open-ended "subscriptions" may eventually become normal for ALL major web-sites and services, including ones whose monopoly powers and commercial support by advertising, have led us to take them for granted. Yes, Microsoft, Google etc, I do include you.
