I have developed a simple SQL-based application to track PPP usage.
SysLog on MikroTik send all PPP & Info messages to server. Retrieve, imports, update automatically according to User, month etc. Automated emailing to users with summary, last few sessions and quota.
There are different methods by using RADIUS / Hotspot etc, but these simple scripts are more than sufficient to keep track off all PPPOE / PPTP connections made to your network.