Chances are that you can connect with most people over Google or Facebook, but it's not perfect. If you are still using their servers, then it might defeat the purpose of avoiding their clients. You can use Off The Record (OTR) to avoid the privacy implications of using the servers with both of them, but none of the official clients support it, so they would have to use an alternative client as well, which defeats the purpose of using existing networks.
Also, there is literally no good foss voip and conferencing options. Jitsi videobridge is the closest thing I've found, and that isn't an XMPP standard and isn't supported by my preferred IM tools. Also, Jitsi is an ugly as sin Java app....
I'm definitely going to try putting aside a week this summer to see how hard it would be to implement videobridge in telepathy. If we could get that and group OTR encryption in it, there would finally be a FOSS communication alternative...