If to store likes in the separate table, it creates one more superfluous query. But at the same time allows:
1. To produce mathematical operations in queries (for an example: SUM, MAX)
2. Adds possibility of usage not only "Likes", but also "Unlike". Now is only "Likes" also anything. System of pluses/minuses.
3. It is possible to select separately a thread (the first post in a thread) and other posts.
4. The extension of the rights for usage likes it is possible to achieve restriction of maximum usage like per hour, per day etc. Also it is possible to disconnect, for example, usage in some thread or a forum "Likes" or only "Unlike". Combinations there can be a set.
5. The aforesaid as a result allows to create rating system.
6. It is possible at review of the list by that at once to select threads which typed the greatest quantity of pluses.
Thus, in the table xf_likes should be stored: