Growing a fan base is slow and monotonous work, if there was a legit fast way without spending a ton of cash everyone would be doing it. To grow your fanbase you have to find forums such as this...but don't be the ******* that gets on, drops a mixtape and never return. Communicate with the members and build a rapport. Send your music to blogs, some will consider posting your work for no money if they like it and some blogs do reviews. I use to send my music to Blockheads blog Phat Friend when he still did demo reviews, had a song reach 1,000 plays on soundcloud in no time. Play shows, do this locally or travel, Slug from Atmosphere credits his success to traveling and touring because they would travel from Minnisota to Texas just to open. Look into college radio stations, or DJ's, etc. and see if they will play your music. If they like it see if you could get an interview.
Now if you have the money...give me some.
lol but for real, you could do email blasts, there are services like istillloveher.de (google it) that will send out an email to their user base with your stuff. Places like that also offer instagram promotion, twitter promotion, etc. But it costs money and don't expect to pay once and be set. Even though this is the fast way it takes months of even this to make a significant impact on your fanbase. Pay to have your music played. Pay to play shows. Money talks in the music industry...almost as much as hardwork.