I guess what you want to know is the retention of users. If that's the case, you can use the same metric (1 post per month, up to 10, more than 10) but look at the data from the signup month perspective (meaning the cohorts). So you would plot the number of users who signed up in, say, September for every month starting from September, do the same for users who signed up in October, etc.

A bar chart won't work that well, I'd suggest using a regular line chart.