FindAGrave.com is not a genealogy website. FindAGrave.com is a genealogy resource.
Anyone can create a memorial or even a cemetery, but should be very careful not to duplicate previous memorials. If you don't live near a cemetery, you can place a request for a volunteer to take a photo of the headstone. The memorial on the right is for my 5x Great Grandmother, Rachel Johnson Albea. The memorial was already online, likely created from a cemetery transcription book, as there was no photo on the memorial. I placed a request and a short time later had a photo of (what was left) of her headstone. I also linked my 4x Great Grandfather, Tilman Albea's memorial to his mother. By doing this I created a small family tree. Through these links, seven generations of my Albea ancestors are connected.
But as I stated, FindAGrave.com is not a genealogy website. The information found in the listings is helpful for genealogy research, but was likely not created for that reason. The memorials are intended to be a listing of a person's burial place and a memorial for them. The fact that this information is beneficial and of interest to genealogists is a side benefit.
But what does the purpose of the website matter, as long as the information is useful? Users need to remember this fact so that they understand how to use the site.
Since the site has been around for so long, odds are that someone else has created listings of your relatives. This great! But it does sometimes cause conflict over "ownership" of memorials. Many people feel possessive of their relatives and want to "control" their ancestor's memorial. It's important to keep in mind that users are entitled to manage the memorials that they created. They took the time to add them to the website, often walking cemeteries for hours. An exception to this is made for memorials that are the direct ancestors within four generations of another user. These memorials, provided that they are not already maintained by a direct descendant, should be transferred to their relative. You can request these memorials be transferred to you by using the "edit" tab at the top of each memorial and clicking on the "Suggest A Correction" link. Before you do this be sure that your request fits the guidelines.
I encourage all genealogists to use the website and contribute to it. Anyone with questions about how to use FindAGrave.com should review the FAQs and check out the message boards.