This is a list of the Facebook groups I have discovered that allow posts from homeowners seeking housesitters and housesitters seeking assignments.
These groups are global so are a good place to start. But also check out my post about Facebook groups dedicated to housesitting in a particular geographical area.
A word of warning. Be very careful about committing to housesitting assignments sourced through these groups. Do your due diligence, take up references, have a video chat and get absolutely clarity on the homeowners expectations and the terms of the sit. Insist on a housesitting agreement with commitments to compensation for travel costs if the homeowner cancels.
Many homeowners will have listings on the big housesitting platforms and are just promoting their listing on Facebook. If so, always use the housesitting platform interface if you can, as most of the platforms have done at least some identity checking and others have a support mechanism in place to assist you if things go wrong.
World Wide Pet & House Sitting

With just over 3000 members as at June 2019, this is one of the biggest groups that is primarily about connecting homeowners and house and pet sitters.
A good place to advertise where and when you are available and to look for homeowners seeking housesitters.
House & Pet Sitting International

This group is one that focuses on offering and finding housesitting assignments and services. You can post your availability as a housesitter, mention where you are looking to sit etc.
A small group currently with around 150 members as at June 2019. But with many of the larger groups not allowing posting of housesitting assignments or services, this group could be a useful one to join.
International House and Pet Sitters

A brand new group that is growing quickly, with almost 1000 members as at June 2019.
It is another group that is about connecting homeowners and housesitters without having to pay membership fees to the housesitting platforms.
Housesit Hustle Community

It’s been pretty quiet in this group just lately, but that might change.
They do have a free course you can download on how to get started in housesitting.