Did you market or promote the community in any way? i.e, is it possible that you were getting less relevant signups from a new source?
It would be nice to get some interpretation on this chart. What info can/could be taken from this chart TV
Essentially this diagram is demonstrating signups and retention for the last 4 quarters. This is a healthy looking community in that the retention rates are relatively good (the bars stay thick) with a slight dip in that rate for people that joined the quarter before last (light green).
Does that help?