Stay connected with all your friends via multiple services like Facebook, Gtalk, Vkontakte, Odnoklassniki, etc. Write and receive messages and SMS, place and receive voice and video calls from your computer. Create and manage a list of contacts, add your own information, and receive alerts when your friends' status changes.
ICQ is an instant messaging protocol and a client. The service is very popular among Russian-speaking users of the Internet, especially in Russia and Ukraine (consequently, as recently as 2010, AOL sold ICQ to a Russian Internet media company). ICQ also connects you to your buddies on AIM and Facebook accounts.
ICQ has been around for over a decade. It has over 100 million users, most of whom reside in Russia and Russian-speaking countries. The service uses the OSCAR protocol, which was pushed by America Online while it owned ICQ. The protocol allows you to communicate with other ICQ users, as well as AIM users, and recently Facebook users.
An ICQ user ought to have a UIN, which is a personal number that identifies the user's record on the server. You can also sign in the service using the email address you provided during the registration. To run the application, you will need a browser that supports Adobe's Air. Most modern browsers support this technology. It is a little but misleading to call it ICQ for Mac as this version can run on any computer that can run Adobe's Air environment. On the other hand, however theoretically impossible, the feature set of the program may vary between Windows or Linux distributions of the program. Regardless, I was able to successfully run ICQ for Mac on Linux.
What are the perks of using this client? Well first of all it is authorized by the owner. Second, it looks nice. I like its interface. For over 10 years ICQ retained its green color theme but changed interface considerably; still, it managed to keep looking very modern and cute.
All you can do in this program is exchange messages with people on your contact list and look up their info and your conversations history. You cannot exchange files or make video calls.
Overall, I do not see the real need to install the application on your computer. There are tons of web services that can connect you to your ICQ friends and also let you exchange files with them, like Meebo. Unless, you like to use authorized freeware software, then yeah, ICQ for Mac is what you are looking for to reconnect with your ICQ, AIM, and Facebook buddies.