Messages, the replacement for iChat, made its debut long before the introduction of Mountain Lion. It first appeared as a downloadable beta version for Lion users. Apple has sent an e-mail to users of that beta version to advise them that time is running out – the company plans to shut the beta down on December 14, 2012. The message reads as follows:

The Messages Beta program for Lion will end on Friday, December 14, 2012. We hope you’ve enjoyed the opportunity to preview Messages.

If you’d like to continue using Messages, upgrade to OS X Mountain Lion from the Mac App Store. Messages is one of many great new apps and features built right into OS X Mountain Lion. Learn More.

Thank you for your participation in the Messages Beta program.

At that point, users who are working with Lion and who depend on instant messaging will need to downgrade back to iChat (losing the additional functionality that Messages provides, like cross-platform support on iOS), or they’ll need to upgrade to Mountain Lion.