I've observed that a couple of times up until now, I didn't make a screenshot for each of these though. What is perhaps noteworthy is that these screenshots were taken with approximately one week of time in between. It seems like sometimes the whole stream gets filled up when somebody signs in using Facebook, and sometimes only one line is displayed, so I have observed both cases. I guess that the correct behaviour is the one in which only one line is displayed for a Facebook login event?
The screenshot does not show the whole filled up member stream, most likely because I noticed it a bit late. (I'm not watching it realtime ;)) But I think it is pretty safe to assume that it occured again.
Instead of tracking down the source of the bug, why not just filter it for now. When you go to add login points, skip the increment if the login occurred within xx minutes of the last login.
<sort of irrelevent aside but an example of "real world" fixes>
On our old AGC/SCADA system from Westinghouse (in service 1983-1998) we had a problem with the display generators (DG). They were (like everything else in that system) custom designed. They would occasionally get into a funny state. All operator console displays would be replaced with "CRT REINITIALIZED" and if the DG reset button in the computer room was not pressed within 30 seconds the entire system would shut down. We always had a backup system running standby which would reboot from backup to online in the event of a failure but that process took over five minutes and on a realtime system controlling billions of dollars of equipment, any outage was a thing to be avoided. Westinghouse coould not find the source of the problem so one of our more ingenious technicians created a work-around. Essentially it was a mechanical finger (a relay) which was triggered when the CRT REINITIALIZED code ran. It automatically did the DG reset.
So sometimes when you can't fix a problem you just have to do something to compensate. That's not ideal, but that's life.