How to index refined null values?
You have two issues here:


large offset: PG will need to scan through the provided offset number of rows and will prefer an index to do this above a table scan.
how to index the data: I think a partial index might help you here. Can you try

create index xxx_idx on latest_channel_snapshots(view_count DESC NULLS LAST) where network_id is null;

See http://www.postgresql.org/docs/9.2/static/indexes-partial.html for more information on partial indexes. In particular, example 11-2 on unbilled orders seems to apply to your case.