A study conducted at University of British Columbia has found that pregnant heifers lick each about 4x more when housed indoors, in contrast to when they are outdoors. The same happened with the number of competition events. Therefore, they speculated that happened due to the closer proximity when heifers are housed in a barn. In this same study, they have found that heifers that spent more time closer to each other they licked themselves more. However, the same did not happen with competition meaning that social licking may work as a "friendship" behavior! So cool!
G. Tresoldi, D.M. Weary, L.C. Pinheiro Machado Filho, M.A.G von Keyserlingk
Social licking in pregnant dairy heifers. Animals, http://dx.doi.org/10.3390/ani5040404