An MSN login is 800 KB, that means every time MSN connects there's 800 KB of data transferred.

The solution is measure it yourself. If you are an experienced user (I'm saying this because I won't tell you step by step how) install IPTraf Monitor. Disable everything that uses the network (even modest editing your .desktop file and rebooting) and connect while looking at the IPTraf screen. Then start your services one by one and check which one is consuming data.

Yappari doesn't consume data at all, unless you're receving a lot of multimedia messages, and even then you just receive thumbnails of around 20 KB in size.