In their corpus, 47.3% of points of interest are annotated invalid, meaning that their methodology to extract points of interest using Foursquare is not very effective.
to their work, we (a) present a corpus with few invalid locations (⇡ 6%), and (b) work with finer-grained temporal information (when somebody tweets, within 24 hours before and after he tweeted, and longer than 24 hours before and after he tweeted).