A computer-implemented method for monitoring a computer system used to support an online network for sharing professional information, the method comprising the steps of: making measurements of the performance of said computer system, said measurements including at least one of throughput measurements and latency measurements associated with requests made to at least one back-end system which is included in said computer system; storing time-series data representing said performance measurements; storing system metrics for the computer system in one or more log files, wherein said system metrics include one or more of central-processing unit usage, memory utilization, traffic to disk, active thread count, garbage collection activity, heap utilization and network traffic; detecting an abnormality in the performance of the computer system from the time-series data by: using a multi-degree polynomial to compute a best-fit line through the time-series data; using the best-fit line to identify a plain in the time-series data, wherein the plain is associated with steady-state behavior for requests made to the back-end system, by detecting, during a transitory startup period of the online network, a start of the plain by periodically looking at the best-fit line representing throughput to determine whether an increase in throughput between consecutive measurements falls below a threshold amount; and detecting the abnormality by examining data points in the plain and looking for a deviation from the best-fit line; looking up the system metrics from the one or more log files which are temporally proximate to the abnormality; and generating a system notification about the abnormality for a system administrator of the computer system, the system notification including the temporarily proximate system metrics to facilitate the operator determining a root cause of the abnormality. making measurements of the performance of said computer system, said measurements including at least one of throughput measurements and latency measurements associated with requests made to at least one back-end system which is included in said computer system; storing time-series data representing said performance measurements; storing system metrics for the computer system in one or more log files, wherein said system metrics include one or more of central-processing unit usage, memory utilization, traffic to disk, active thread count, garbage collection activity, heap utilization and network traffic; detecting an abnormality in the performance of the computer system from the time-series data by: using a multi-degree polynomial to compute a best-fit line through the time-series data; using the best-fit line to identify a plain in the time-series data, wherein the plain is associated with steady-state behavior for requests made to the back-end system, by detecting, during a transitory startup period of the online network, a start of the plain by periodically looking at the best-fit line representing throughput to determine whether an increase in throughput between consecutive measurements falls below a threshold amount; and detecting the abnormality by examining data points in the plain and looking for a deviation from the best-fit line; using a multi-degree polynomial to compute a best-fit line through the time-series data; using the best-fit line to identify a plain in the time-series data, wherein the plain is associated with steady-state behavior for requests made to the back-end system, by detecting, during a transitory startup period of the online network, a start of the plain by periodically looking at the best-fit line representing throughput to determine whether an increase in throughput between consecutive measurements falls below a threshold amount; and detecting the abnormality by examining data points in the plain and looking for a deviation from the best-fit line; looking up the system metrics from the one or more log files which are temporally proximate to the abnormality; and generating a system notification about the abnormality for a system administrator of the computer system, the system notification including the temporarily proximate system metrics to facilitate the operator determining a root cause of the abnormality.