Once you have around 10 to 15 newcomers joining a week, you can’t publicly welcome each of them. It wouldn’t help newcomers feel a part of the group (“oh gee, I’m one of 50 people being mentioned today!”). It’s tedious for existing members.
I’m betting the newcomer-welcome posts are the ones you’ve already begun to ignore. There’s no evidence that this leads to long-term participation anyway.
A better approach is to welcome them within an existing group.
This takes an extra few minutes.
Lookup a newcomer’s profile, Google their name and e-mail address. You should be able to find some details about them. Usually a Twitter account, Linkedin profile or other information.
Look specifically for their location, the type of work/variation of their hobby, and any unique quirk.
Now instead of giving them a public welcome, introduce them to a group of other people like themselves. For example, if the newcomer is from Austin. Send a private e-mail/group message to members from Austin to welcome the newcomer. Other members can now quickly reply to say hi.
(locations are especially easy for this. Mailchimp lets me pull a list of members specifically for this).
Better yet, ask the Austin group if anyone would like to greet future new members from Austin and send the names across as they come in.
You can create an excel spreadsheet or segmented list to add each newcomer by location or work variant. Soon you will find you can quickly introduce newcomers to existing groups without spamming every member in the community.
It’s far more fun and effective than another monotonous public welcome post.