What is a NetSplit?
The basic definition of a netsplit is "when two or more linked servers on a network delink". The most common sign of a netsplit occurring is when a sizable chunk of the members in all channels simultaneously quit all with the same reason. Usually, when the servers get relinked, all those who quit rejoin, unless they quit on that server during the netsplit.
Where did my friends go?
When a netsplit occurs, any communications to people on that server are not possible without joining that specific server. Usually, netsplits do not last long. We do not recommend that you rapidly quit one server and join the other server during a netsplit, as conditions are rapidly changing during this instant in time.
What does a netsplit look like to chat?
The message presented is different from each user server, but this example from the US server shows a unique quit message containing two server names. This is an indication of a netsplit.
* Hercule (Mibbit@Hercule.staff.mibbit.net) Quit (irc.us.mibbit.net irc.uk.mibbit.net) * Sindacious (email@example.com) Quit (irc.us.mibbit.net irc.uk.mibbit.net) ..etc
The first server is the server you are on. The second server is the server that split.
- History was made on Feb 21, 2010, at 03:55GMT when the US and UK Mibbit servers netsplit for the very first time, lasting under two minutes. As expected, the healing was automatic. Knock on wood. 
[Feb/21 03:55] -irc.us.mibbit.net- *** Notice -- No response from irc.uk.mibbit.net, closing link [Feb/21 03:57] -irc.us.mibbit.net- *** Notice -- link irc.us.mibbit.net -> irc.uk.mibbit.net established