The swimming community strives to be an area where we can all enjoy our swims in a safe manner.
However we are also an online community so please avail yourself of the the general rules regarding social etiquette and safety including your privacy settings on sites such as this https://www.bbc.co.uk/cbbc/findoutmore/help-me-out-staying-safe-online
In particular do not arrange meet someone in real life for a swim if you have only met them online unless they are vouched for by a mutual friend or you take a friend with you.
Facebook also has privacy tools such as blocking and restricting the audience for your personal posts which can be used.
If you have concerns then please report to the admins of your group who will consider the action necessary and the police can be emailed on the 101 service.
That's not to say we can't enjoy making new friendships - just that , as in all things online, we need to stay sensible and safety aware.