I think google might be having a hard time indexing your web pages, 50.000 is alot. So my suggestion would be breakdown your sitemap into pieces like so
" rel="nofollow" target="_blank">http://www.sitemaps.org/schemas/sitemap/0.9">
If you breakdown you will have a better luck of having those 50.000 urls indexed.
Sitemaps.org explanation of the issue
¬†¬†¬†¬†You can provide multiple Sitemap files, but each Sitemap file that you provide must have no more than 50,000 URLs and must be no larger than 10MB (10,485,760 bytes). If you would like, you may compress your Sitemap files using gzip to reduce your bandwidth requirement; however the sitemap file once uncompressed must be no larger than 10MB. If you want to list more than 50,000 URLs, you must create multiple Sitemap files.
¬†¬†¬†¬†If you do provide multiple Sitemaps, you should then list each Sitemap file in a Sitemap index file. Sitemap index files may not list more than 50,000 Sitemaps and must be no larger than 10MB (10,485,760 bytes) and can be compressed. You can have more than one Sitemap index file. The XML format of a Sitemap index file is very similar to the XML format of a Sitemap file.