I was tempted to get a bunny from a fb page instead of from a shelter because I thought along the lines of 'at least the shelter bun is safe, who knows where the fb buns are going to end up'. Now I am so glad I resisted and went to a rescue instead! The people on these sites buy pets like accessories and then sell them on as soon as they are grown so they can buy more babies. I saw one woman who bought a jack Russell and a cat, then gave them away free so she could buy a new puppy (I got the impression her landlord had a two pet policy or something...) and then I saw her commenting on another post about buying more animals.
Other people that are like '£10 for my rabbit, good home wanted' and then you see them on another page posting under posts about buying a hamster or guinea pig...
If you buy the animals off there you fuel their ability to buy and abuse more, whilst by going to a shelter you help support the shelters ability to rescue more. I finally had to leave the pet pages after a week where I got into a massive argument with a woman who was selling a 2 foot indoor cage for 2+ guinea pigs, someone else who was giving their rabbits away for free and I politely told them they could be used for bait and directed her to a rescue and they told me it was none of my business, and seeing several more staffie litters being advertised when their are so many pups rotting in shelters atm
It breaks my heart