i think if they integrate google+ fully inside their android os they will have one big advantage that facebook will not be able to compete with. i think they have the potential to beat facebook if they want to definitely.
Totally diffrence between FB and G+.
FB for all email accounts, peoples come to share the fun things, don't care more about the other thing, just concentrated friends, fun things and some time is politically.
G+ just for Gmailer only, i reconized that most of peples get in there with business purpose. Almost the G+ invited to my email is the company name, if some one has automatically invited with business purpose also. This is my thought, hope it help.
Google as such is known more for its Search Engine and its web tools rather than anything else on the other hand Fb being first off the block has had a phenonemal exposure in the same category & people can't co-relate google with anything much similar.
I don't think google + will take over FB. because as far as I know google has started google + for some ranking factor add to his algorithm to ensure the quality of good content, like he has added author ranking in algorithm update and google is pushing writers to complete their google profile and create +page to ensure the quality of the content. for example how many people push the plu button for a specific writers content. on The other Hand FB is like Google he always adapting new updates and ideas to implement.