Hi, i like this project, i suggest do a really good platform with strong back end, i don't recommend to you use any interpreted language(like to php, python, ruby on rails or nodeJS i love NodeJS but no here) to back end, we will need need keep real time connection with the clients (web, browser, futures mobile applications and more) implementing web sockets, we need good performance with compiled language like to java, scala, c++, go or dart, additionally will need a real time system to intercommunicate services from many language, i recommend apache kafka for this, kafka have really good performance(i think have the better performance in real time applications), this is a real-time queue system with data persistence.
We need implement Apache sorl or ElasticSearch (i think is better elasticSearch) to do complex search and find in instant-time.
Additionally will need a good database, i recommend use mongodb or cassandraDB. A Sql databse like to Mysql, postgresql or oracle DB is no better option for a big project.
Maybe but yet, will need to implement hadoop.
Don't worry by my English i have bilingual people on my team.
I recommend to you don't trust to me, but yes read about this technologies suggested.
If you have any question, just contact to me and ill help you, bye and have good day.