Not supporting Facebook (I don't have an account or use it). But supporting a Facebook login on your site isn't necessarily indicative of anything. Facebook logins, like Google etc. are using OpenID, so by providing Facebook login support, you're essentially just supporting OpenID logins, which is fine in of itself. Once you support one, you support them all, it's just a case of giving your login page the options and URLs for each OpenID provider.
There is of course a risk: That is can you trust the OpenID provider? Facebook could potentially use the tokens issued to access your site services as the user they authenticated and pillage their info.
So agreed, I wouldn't want Facebook specifically supported on my site, but continuing to support OpenID is a good thing as - if we ever do get a P2P / Nextcloud style social network, OpenID will allow each user to authenticate themselves to each-others "clouds". It would also let you login to 3rd party sites with your personal cloud used to authenticate you - never exposing your username, password or personal details to the 3rd party site.
The fact these sites support a Facebook login, means they can easily be switched to alternative OpenID providers.
This has a nice diagram of how OpenID works - scroll down to overview and follow the steps below the diagram. The 3rd party site would be the relaying party (Web app server), Facebook the OpenID provider: