Lack of revenues was an issue for Facebook, said sources, especially related to fees Twitter pays for delivery of its messages to cellphones.

While the issue has been manageable in the U.S., Twitter cut off its SMS support in some international markets this summer because of too-high costs.

But, if Twitter was offered to Facebook’s 120 million users, Facebook execs estimated that it might have to deal with huge SMS fees–up to $75 million annually.