ORDER BY optimization for a large table

I am currently aggregating posts from different social networks. Currently I have 50-100 million posts from Facebook, Twitter, Youtube, Instagram , Pinterest.

Consider a table posts

posts{    id  int(11),    user_id  int(11),    url varchar(256),     image varchar(256),     source int(11),  // Social Network Source     created bigint(20), // Publish time of the post     visible int(1) // Public or Private Posts}

Using InnoDB

Indexes :

primary key on iduser_id,sourceuser_id,created

Space usage

Type    Usage Data    45,876.0    MiBIndex   4,959.0 MiBTotal   50,835.0    MiB

Using Amazon RDS with 7.5 GB RAM.

The queries I am executing are as follows

select id from posts where user_id={user_id}   and visible=1  order by created desc   LIMIT 20 ;select id from posts where user_id={user_id}   and source={your_network}   and visible=1 order by created desc   LIMIT 20 ;

For users with > 500k posts sometimes my queries take a really long time.

I have run explain and you can see the results below

EXPLAIN SELECT idFROM  `posts` WHERE user_id =123529745 and source=1 and visible=1ORDER BY created DESC LIMIT 20id    select_type       table   type    possible_keys   key     key_len ref     rows    Extra 1       SIMPLE          posts   ref     user_id_2,user_id_4,user_id_3,created   created 4       const   954174  Using where; Using indexid    select_type       table   type    possible_keys   key     key_len ref     rows    Extra 1       SIMPLE          posts   ref     user_id_2,user_id_4,user_id_3,created   created 4       const   742308  Using where;

Can you please suggest me any optimizations?