I don't know if italki has such a list, but they do seem to rank-order languages in the "search for a language partner" option. Judging from that list, the most commonly sought languages are, in order: 1) English, 2) Mandarin, 3) French, 4) Spanish, 5) Portuguese, 6) German, 7) Japanese, 8) Korean, 9) Arabic, 10) Hindi, 11) Italian, and 12)Russian. Intuitively, this ordering makes sense to me. People doing business internationally probably prefer to learn English and Chinese because of the economic supremacy of these two nations.