MySpace and Facebook may be frenemies forever, but now they have a new mutual friend that will pass notes between them. Meebo, the Web-based IM service, now supports IM accounts from both social networks. That means you can enter your Facebook and MySpace account IDs into Meebo and the chat with friends from both social networks in one IM box. The integration will work on Meebo.com itself, as well its Community IM partner sites.

While the partnership with MySpace is official, Meebo basically reverse-engineered Facebook’s IM. So if Facebook decides to change its IM protocols, the Meebo integration could break until its engineers apply a band aid. The MySpace integration, in contrast, is built on top of official APIs and incorporates all the security features of MySpace IM.

Meebo also supports AIM, Gtalk, MSN Messenger, Yahoo Messenger, Jabber, and ICQ accounts. It claims 40 million members, who send 5 billion messages a month. ComScore reports 7.4 million monthly visitors worldwide to Meebo.com, and shows some flattening of growth. Tapping into Facebook and MySpace should help Meebo keep its growth going.