From the "Robert's Rules of Order" website... http://www.robertsrules.com/faq.html#4

Question 6:
Do abstention votes count?

Answer: The phrase "abstention votes" is an oxymoron, an abstention being a refusal to vote. To abstain means to refrain from voting, and, as a consequence, there can be no such thing as an "abstention vote." In the usual situation, where either a majority vote or a two-thirds vote is required, abstentions have absolutely no effect on the outcome of the vote since what is required is either a majority or two thirds of the votes cast. On the other hand, if the vote required is a majority or two thirds of the members present, or a majority or two thirds of the entire membership, an abstention will have the same effect as a "no" vote. Even in such a case, however, an abstention is not a vote. [RONR (10th ed.), p. 387, l. 7-13; p. 388, l. 3-6; p. 390, l. 13-24; see also p.66 of RONR In Brief.]