I don't think it's FP+ causing the longer lines, it sounds like the park is just more crowded. Assuming that the same number of FP+ are distributed per hour as FP- (as I would expect it to work this way and also as you found FP+ had "sold out" when you tried to make new selections) the FP+ line shouldn't slow the standby line down any more than FP- did.
We don't have any figures, but I wouldn't imagine the crowds are much larger, in fact someone was saying that crowds should have been lighter recently due to a later mardi gras which usually makes the time around Presidents day week very heavy ... that was not the case this year.
There have also been some reports that they are giving out MORE FPs under FP+ than they did under FP-, and that more people are using them. That stands to reason that SB lines would back up a little more and we have clearly seen FP lines backing up more ...



