Fall Out Boy is a band that has made fantastic music for years now. From when I am posting this (2015) they have released two albums (Save Rock & Roll and American Beauty/ American Psycho). They have had appearances on shows like Ellen and Jimmy Kimmel. They've had top ten singles and one of their songs (Centuries) went platinum. Also, AB/AP was the #1 album in the US when it was relased.
When things are hyped in the media or on social websites they tend to have initial success if they ever had a large fan base. Fall Out Boy's music is obviously liked by many as is their style and genre. Continued success will come if they continue to please their fans and that will require some ingenuity.
My opinion of the band aside (tiresome within minutes of hearing it), I think this will initially be successful. It's fallen apart before and it will again, but it will have that initial surge of fans flocking to it once given another opportunity to do so. It'll fizzle out probably pretty quickly.
I do not believe that Fall Out Boy's reunion will be successful. A very small percentage of musical reunions have worked over the years since band have to stay in the spotlight to remain relevant. I think that they had their time in the spotlight and a reunion will be overblown and short lived.
The Fall Out Boy reunion will not successful. People get nostalgic about music and set expectations of seeing bands again way too high. I truly believe most people are going to be disappointed. It happens with so many bands and groups, Fall Out Boy will be no different from the rest.